LONDON — U.S. elections are rarely won on foreign policy — presidents enter the Oval Office focused on changing America, not the world.
But many presidents end up being defined by their foreign policy decisions.
Donald Trump's first priority will be to deal with the challenges to American dominance posed by China and Russia. And, possibly, avoid war with either of them.
The U.S. remains the world's most important economic and military power, but a resurgent China and Russia are determined to remake the rules that have guided the globe since 1945.
These challenges come as a post-World War II world order shaped largely by the United States begins to crack.
Russia is testing NATO, the U.S.-led military alliance. China is demanding influence to match its economic might. Meanwhile, Europe's unity is crumbling, with nationalism on the rise. Democracy is being questioned. Islamist militancy is a global danger. Nuclear proliferation threatens. And climate change is melting the Arctic, and opening a new continent to potential conflict, as Russia and others lay claim to swaths of the continent.
The dizzying array of challenges a new administration faces prompted former White House national security adviser Zbigniew Brzezinski to offer this piece of advice: "More caution in public statements but greater clarity about U.S. commitments around the world should be high on next president's to-do list."
Trump, who ran an "America First" campaign while also making overtures to Russian President Vladimir Putin, has no foreign policy experience so will have to learn fast.
Trump will begin by prioritizing domestic concerns, but the world will bang on his windows — almost immediately in the case of Syria, ISIS, Russia and North Korea.
He inherits an America less mired in war than at the start of President Barack Obama's time in office but still deeply engaged in military intervention across the world. In 2016, the U.S. carried out airstrikes in seven countries from Somalia to Syria and conducted special operations missions in many more.
Voters tell pollsters they want their new president to concentrate on American problems, but they're also clear on other priorities including the fight against ISIS, keeping NATO strong and ensuring America is the world's major power.
In an unstable world, tripwires are everywhere — Ukraine, Estonia. Kaliningrad, North Korea, the islands of the South China Sea. Allies are less reliable than they once were. Britain has lost its military appetite and is consumed by its exit from a faltering European Union.
Europe is beset by problems — not only Brexit, but slow growth and a currency crisis and has "lost its capacity to think strategically," according to analyst Philip Stephens.
In Asia, the decision of Philippine President Rodrigo Duterte to side with China "economically and militarily" and turn its back on America could be a one-off, or it could be a taste of things to come across the region.
Trump faces a world where enemies seek to take advantage of perceived Western weakness, where power is being flexed without regard to punishment and cyber warfare is getting serious.
The U.S. is still recovering from a massive economic downturn that has profoundly dampened voters' tolerance of foreign entanglements. And the divisions and anger exacerbated by a bitter election campaign will take years to heal, and may sap his energy for foreign initiatives.
As ever, so many of the world's challenges are interlinked. Russia and Iran are deeply involved in the Syrian war, supporting President Bashar al-Assad versus the U.S. and its allies. Any solution to North Korea's nuclear program depends on China. And climate change involves everyone on the planet.
But the biggest questions are the ones he may have little time to think about once his long days as president begin. They are about American leadership, or the lack of it, in the 21st century.
Does the U.S. still have global responsibilities? Does that mean being the global policeman? What is it prepared to fight for? Will it pay any price to uphold its values? Is the U.S. truly that "shining city upon a hill" that presidents like John F. Kennedy and Ronald Reagan proclaimed it to be?
He inherits an America increasingly hesitant about using its power.
In Syria, it seemed reluctant to back up its promises of action with force, if the White House's refusal to punish Assad's government for allegedly using chemical weapons is any guide.
Obama's "pivot" towards Asia seems more rhetoric than reality. And can it really afford to downplay its role in Europe, or the Middle East? It hoped to extricate itself from the latter, in order to boost its prospects in China's hinterland, but the chaos that followed the Arab Spring and Russia's resurgent foreign policy have guaranteed that America's troops and attention remain in old battlegrounds.
These are the known hazards. Then there are what former British Prime Minister Harold Macmillan feared the most: "Events."