Western leaders are embarking on an extraordinary stretch of summitry this week, which could give them a chance to project unity to adversaries who increasingly view the West as something to be defied, disregarded or even repudiated.

But as they gather on the windswept bluffs of Normandy for the 80th anniversary of D-Day on Thursday, and five weeks later in Washington for a NATO summit, the leaders will be tested across a range of divisive issues: wars in Ukraine and Gaza, the rise of China and, perhaps most daunting, the future of the United States.

On one level, D-Day and NATO are inspiring bookends: the first, a nostalgic commemoration of the Allied victory over Nazi tyranny; the second, a 75th birthday party for the alliance that grew out of the ashes of World War II. In between, there is a Ukraine peace conference in Switzerland and a Group of 7 leaders’ summit in Italy.

Yet beneath the pride and pomp, there will be nagging doubts, not least about the direction of American politics. President Biden will travel to France and Italy (he is expected to skip the Switzerland forum), but he is squeezing in the diplomacy amid an election-year battle against former President Donald J. Trump, whose victory in November would call into question the very survival of the alliance that Western leaders are spending so much time celebrating.

Mr. Trump’s shadow over the events is inescapable. The summits are bookended by his convictions last week for falsifying business records and his sentencing, which is scheduled for July 11, the final day of the NATO meeting in Washington. That will provide European leaders with a front-row view of America’s political dysfunction, even as Mr. Biden strives to unite them against the Russian threat.