What happens if US withdrawal from the world stage is irreversible?

Today’s international security challenges—whether related to the Middle East, global terrorism, Russia, or North Korea—are far more troubling than the crises of recent decades. This is not because their scale and scope are dramatically different from previous ones. Rather, they pose a greater threat to global stability because the United States, along with partners such as the United Kingdom, are not providing the necessary leadership to manage and mitigate them.

While Whitehall may be distracted by Brexit, President Trump has…