Speaking from the Oval Office last month as he explained his decision not to seek re-election, President Biden boasted about his accomplishments. One of them, he suggested, was presiding over an era of peace.

“I’m the first president in this century to report to the American people that the United States is not at war anywhere in the world,” Mr. Biden declared to the nation.

But while America is no longer waging a large-scale ground war like those in Iraq and Afghanistan, for much of his tenure Mr. Biden has seemed like a wartime leader.

Since pulling the last American troops out of Afghanistan three years ago, Mr. Biden has spent much of his presidency mobilizing public opinion and military might against Russia’s invasion of Ukraine and playing a deeply engaged role in supporting Israel in its war in Gaza, and against Iran and the groups it backs.

“War will be a key part of Biden’s legacy,” said Stephen Wertheim, a senior fellow at the Carnegie Endowment for International Peace. “What’s hard to wrap one’s head around is that, although the United States is not directly involved in the wars in Ukraine or Gaza, the risks of large-scale conflict have become higher over the course of Biden’s presidency.”

The implications for Mr. Biden’s successor — either former President Donald J. Trump or Vice President Kamala Harris — could be huge.