As NATO leaders gather in Washington starting Tuesday, they will celebrate the strength of their alliance on its 75th anniversary while confronting deep uncertainty about its future.

In recent years, Russia’s aggression against Ukraine has given NATO, founded after World War II to defend Europe from the Soviet Union, a renewed sense of purpose. But the alliance also faces grave threats, including from right-wing skeptics who are gaining power in nations such as Germany and France.

And the potential return to the White House of Donald J. Trump, who has derided NATO and even mused about withdrawing the United States from the alliance, has raised alarms among its members.

Here’s what to watch for during the three days of NATO meetings in Washington this week.

Staring Down Putin

Perhaps the summit’s most important goal will be sending a signal of unity and strength to Moscow.

Officials say that President Vladimir V. Putin of Russia is counting on NATO’s efforts to counter his aggression to dwindle, potentially allowing him to conquer much more of Ukraine and even turn his sights to other nations.

That’s why a central theme of the summit will be demonstrating not only a long-term commitment to Ukraine, but also the endurance of NATO itself.