Commander-in-chief in waiting? Hillary Clinton is optimistic about America’s ability to defend its interests abroad ©Justin Sullivan/Getty Images

When Hillary Clinton eviscerated Donald Trump in a speech in June, she did so on the grounds that he was hopelessly unprepared and temperamentally unsuited to be commander-in-chief. She derided the foreign-policy positions of her Republican rival for the presidency as a farrago of contradictions and provocations—“not even really ideas,” she said, “just a series of bizarre rants, personal feuds, and outright lies.”

What Clinton didn’t do was to lay out her own foreign-policy agenda. Her implication was that Trump was so manifestly unqualified for the Oval Office that her prescriptions for how to end the civil war in Syria or counter the predations of Vladimir Putin in Ukraine were almost beside the point. The reaction from the cheering crowd in San Diego, California suggested that Clinton was right: she didn’t yet need to offer a lengthy list of policy alternatives to Trump. Her record—as First Lady to President Bill Clinton, Senator from New York, and above all, as President Barack Obama’s secretary of state—spoke for itself.

Still, as American voters pivot this summer from the primaries to the general election, Clinton’s foreign policy will come under greater scrutiny. When it does, people might be surprised by the extent to which she parts company with her former boss. A fervent believer in the concept of “American exceptionalism,” Clinton is more open than Obama to the calculated use of military force to defend national interests. She is more optimistic than he is about American intervention, believing that it does more good than harm. She believes the writ of the United States properly reaches, as George W Bush once declared, into “any dark corner of the world.”

Clinton and Obama, one could argue, have come to embody competing visions of America’s role in the world: his vision is restrained, inward-looking, radical in its acknowledgment of limits; hers is hard-edged, pragmatic, unabashedly old fashioned.

Those long-held principles will also put her at odds with Donald Trump. If one picks through the bluster and contradictions in his statements, there are some clear warnings. He has threatened to withdraw American support for NATO, pull the US nuclear security umbrella from over East Asia and shun the nation-building of the Bush years. Indeed, the 2016 election could scramble the traditional dynamic between hawkish Republicans and dovish Democrats. This time, it is the Democrat whose foreign policy could reassure mainstream Republicans—at least when compared to the neo-isolationism and “America First” rhetoric of Trump.

To understand Clinton’s foreign policy, it helps to study her four years as a member of Obama’s cabinet. Loyal, disciplined and determined to be a team player, she rarely, if ever, showed that there was daylight between her and the president. For journalists such as me, who expected the kind of withering sniper fire between the US Department of State and the Oval Office that Clinton and Obama had exchanged during their 2008 primary battle, the display of unity was stifling.

But to travel with the Secretary of State as I did as a correspondent, to 43 countries on five continents, was to witness a woman completing a decade-long metamorphosis, one that widened, rather than narrowed, her differences with the president she had agreed to serve. Clinton was shedding the last vestiges of her image as a polarising, left-wing social engineer in favour of a new role as a commanding figure on the global stage, someone who could go toe-to-toe with the mullahs in Tehran or the cold warriors in Moscow. A loyal lieutenant, yes, but a general in waiting.

Under the surface, Clinton’s Manichean world view was always there. It turned up early, in her blunt prediction to an Arab foreign minister that the Iranians would spurn Obama’s offer of an olive branch. Later, one could see it in her support of military commanders in their request for a larger American troop deployment to Afghanistan than even the Republican defence secretary wanted. Or in her support of the Pentagon’s recommendation to leave 10,000 to 20,000 troops behind in Iraq. It surfaced in her campaign for air strikes in Libya to prevent a slaughter by Muammar Gaddafi, and it fuelled her case, the summer before she left the State Department, for funnelling weapons to the rebels fighting Bashar al-Assad in Syria.

Avidly, if discreetly, Clinton played the house hawk in Obama’s war cabinet.

Once she was a private citizen, with the presidency again in her sights, the fissures between them became harder to conceal. Nor was she inclined to do so. She came out against his ambitious Asian trade pact, after having been one of its most enthusiastic advocates. She began to etch clear policy differences with Obama on Syria and Russia, a distancing his aides found opportunistic, if unsurprising. In August 2014, Clinton said Obama’s refusal to arm the rebels in Syria left a security vacuum there and in Iraq, which had been filled by the brutal warriors of Islamic State (IS). Her criticism antagonised a president who already felt embattled.

Clinton’s break with Obama over Russia played out similarly. She had long been more suspicious of Putin than the president. At a Democratic fundraiser in California in March 2014, she likened his annexation of Crimea to Hitler’s conquest of the Sudetenland in the 1930s. Eighteen months later, she said Obama’s restrained response to Putin’s bullying of Ukraine was inadequate. And when Putin intervened in Syria on behalf of Assad, Clinton sounded the trumpet of a new Cold War. “All the Russian experts that thought that their work was done after the fall of the Berlin Wall,” she said, “I hope that they will be dusting off their materials.”

Their differences surfaced again during the bloody months at the end of 2015, when Islamist militants carried out killing sprees in Paris and California. The carnage propelled terrorism to the forefront of yet another presidential campaign. Suddenly, the tangled conflict in the Levant was no longer just a riddle for foreign-policy experts; it posed a direct threat to the homeland, throwing Obama on the back foot and playing out in the crude appeals to nativism and nationalism by Trump and other Republican candidates. Syria was where Clinton had first split with him over supplying arms to the rebels; now they split again over her call to impose a no-fly zone over northern Syria, as her husband had done in Iraq in the 1990s to protect the Kurds.

“Look,” she said at the Council on Foreign Relations a few days after the Paris attacks, “I have made clear that I have differences, as I think any two people do.”

Clinton still embraced central tenets of Obama’s foreign policy; it was, after all, her foreign policy, too. In the autumn of 2015, she articulated the case for his much-disputed nuclear agreement with Tehran to an audience at the Brookings Institution. But their public remarks only underscored how differently Clinton viewed the achievement than Obama. He called it “the strongest nonproliferation agreement ever negotiated.” She called it a flawed deal worth supporting only if it was linked to relentless enforcement, a concerted effort to thwart Iranian malfeasance in the Middle East, and an unwavering threat to use military force to prevent Iran from ever getting a bomb. “My starting point will be one of distrust,” she said.

Dennis Ross, a former aide to Clinton and Obama who played a role in the secret negotiations with Iran, said: “It’s not that she’s quick to use force, but her basic instincts are governed more by the uses of hard power.”

The differences between Clinton and Obama are not ideological as much as generational, cultural, even temperamental. She is a Midwesterner, a product of the Cold War who came of age during the Vietnam era and watched as her husband articulated a new rationale for humanitarian intervention in the Balkans in the 1990s. She is a woman aspiring to a job that has been held only by men. Obama is a child of the Pacific Rim who came of age after Vietnam and had no first-hand exposure to the Balkans campaigns. The formative foreign-policy event of his lifetime was the American misadventure in Iraq.

While Secretary of State, Clinton stuck close to Obama but she has since distanced herself from him on Russia and Syria ©Larry Downing/AFP/Getty Images

Obama came into office as a counter-revolutionary, seeking to end Bush’s wars and restore America’s moral standing. But his ambitions were even larger than that: he set out to reconcile Americans to a world in which the US was no longer the undisputed hegemon. He shunned the triumphalist language of American exceptionalism, declaring that the nation’s unique character lay not in its perfection but in its unending struggle to live up to its ideals. He refused to be drawn into distant conflicts, with the much-regretted exception of Libya.

Clinton is more conventional and more political. Her foreign policy is less a doctrine than a set of impulses, grounded in cold calculation and what one aide calls “a textbook view of American exceptionalism.” She is at heart a “situationalist,” somebody who reacts to problems piecemeal rather than fitting them into a larger doctrine. Her flexibility has led people to read different things into her foreign policy: Republicans accuse her of being an Obama retread; Obama loyalists grouse that she dramatised her divisions with the president on Syria and Russia for political reasons; Leslie Gelb of the Council on Foreign Relations warns that she could end up in thrall to the neo-conservatives who led the US into Iraq. “She takes the position that leaves her the least vulnerable,” he said.

“Obama views adversaries in terms of their grievances; Clinton views them in terms of their interests”

Those characteristics make her a ready warrior but a cautious diplomat. Unlike most modern-day secretaries of state, Clinton kept her distance from peace negotiations between the Israelis and Palestinians, judging them to be an uphill climb and not worth the risk of alienating Jewish voters at home. Obama made daring overtures to Iran and Cuba; it’s not clear the US would have achieved either, had Clinton been elected president in 2008 instead of him. Obama’s statesmanship flowed from a very different source than Clinton’s: he tended to view adversaries in terms of their grievances towards the US; Clinton views them more traditionally, in terms of their interests. “It leads you in a different direction,” Dennis Ross said.

Predicting how a secretary of state would act as commander-in-chief is, at some level, a fool’s errand. The last person to make the transition was James Buchanan in 1857; his presidency, which accelerated the slide towards the Civil War, was widely judged the greatest failure in the history of the Republic. Clinton might view the diplomatic stakes differently as president than she did as secretary of state. Militarily, she would face the same constraints Obama did, not just at home but abroad. The breakdown of the 20th-century American order, Obama’s defenders note, has made the world less amenable to any president’s efforts to control it.

“If you look at Obama and his rhetoric in 2008, you would have expected a transformational and maximalist president,” said Joseph Nye, the Harvard political scientist. “He was going to ban nuclear weapons. He was going to repair relations with the Muslim world. We were going to have a reset with Russia. These were ambitious goals, but he turned out to be a rather prudent retrencher. The pendulum is going to swing back somewhat now, and Hillary Clinton is probably going to be less of a retrencher. The question is how much leeway she’ll have.”

It was the second Thursday in December 2015, 53 days before the Iowa caucuses, and Jake Sullivan, her top policy adviser, was sitting down with me in Clinton’s headquarters in Brooklyn, to explain how she was shaping her message for a campaign suddenly dominated by concerns about national security. Clinton’s strategy, he said, was two-fold: first explain to voters that she had a clear plan for confronting the threat posed by Islamist terrorism; and second, expose the Republicans as utterly lacking in experience or credibility on national security.

“There’s no doubt that Hillary Clinton’s more muscular brand of American foreign policy is better matched to 2016 than it was to 2008,” said Sullivan, who had been at her side in all 112 countries she visited as secretary of state.

In a series of policy speeches last autumn, Clinton had reasserted her hawkish credentials. She said the US should consider sending more Special Operations troops to Iraq than Obama had committed to help the Iraqis and Kurds fight the jihadi warriors of IS. She came out in favour of imposing a partial no-fly zone over Syria, which he opposed. And she described the threat posed by IS to Americans in starker terms than he did. As was often the case with Clinton and Obama, the differences were less about direction than degree. She wasn’t calling for ground troops in the Middle East, any more than he was. She insisted her plan was not a break with his, merely an “intensification and acceleration” of it.

There were good reasons for Clinton to let her inner hawk fly. Americans were more worried about an attack on the homeland than at any time since 9/11. A CNN/ORC poll taken after Paris showed that a majority, 53 per cent, favoured sending ground troops to Iraq or Syria, a remarkable shift from the war-weary sentiment that prevailed during most of Obama’s presidency. Republican candidates reached for apocalyptic metaphors to demonstrate their resolve. Trump called for the US to ban all Muslims from entering the country “until we are able to determine and understand this problem and the dangerous threat it poses.”

Yet such spikes in the public appetite for military action tend to be transitory. Three weeks later, the same CNN poll showed an even split, at 49 per cent, on whether to deploy troops. Trump does not favour major new deployments of American soldiers to Iraq and Syria. If anything, he is more sceptical than Clinton about intervention and more circumspect than she is about maintaining the nation’s post-war military commitments. He loudly proclaims his opposition to the Iraq War, for example, even if he once expressed support for it.

In showing her stripes as a prospective commander-in-chief, Clinton will no doubt draw heavily on her State Department experience, filtering the lessons she learned in Libya, Syria and Iraq into the sinewy worldview she has held since childhood.

How well Clinton’s hawkish instincts match the country remains an open question. Americans are weary of war and are now suspicious of foreign entanglements. And yet, after the retrenchment of the Obama years, there is evidence they are equally dissatisfied with a portrait of their country as a spent force, managing its decline amid a world of rising powers, resurgent empires and lethal new forces such as IS. If Obama’s minimalist approach was a necessary reaction to the maximalist style of his predecessor, then perhaps what Americans yearn for is something in between—the kind of steely pragmatism that Clinton has spent a lifetime honing.