WASHINGTON — The old adage about the interlude between an election and an inauguration is that there is only one president at a time.

Try telling that to the rest of the world now.

While one president, the one actually still living in the White House, attends international summit meetings and brokers a Middle East ceasefire to cap his tenure, another president, the one who has not actually taken office yet, is busy conducting a foreign policy of his own from his Spanish-tiled Florida estate.

Without waiting to be sworn in, President-elect Donald Trump effectively declared a trade war last week by announcing he would impose tariffs on America’s friends, Canada and Mexico, as well as its rival China on Day One of his administration. The next day, President Joe Biden strode into the Rose Garden to announce an agreement to end more than a year of fighting between Israel and Hezbollah.

This is America in the time of transition, making peace and declaring war, all in the same 24-hour news cycle — two presidents leading the country in two different directions, one officially, the other unofficially; one representing the past and present, the other the future. Whipsawed and maybe just a little confused, foreign leaders are left to calculate whether it makes sense to try to get something done with the outgoing leader or brace for the reality of his successor.

“Transitions always result in a momentum shift to the new team, but this time around feels more pronounced than any transition in recent memory,” said Suzanne Maloney, director of the foreign policy program at the Brookings Institution in Washington and a former State Department adviser.

“Leaders in capitals around the world are trying to take advantage of the moment to try to curry favor with Trump himself at a time when it may still be possible to shape his agenda,” she added. And Trump’s “inner circle is dismissive of the traditional Washington protocols that might suggest some discretion during the transition.”

The tariff threats may just be an opening bargaining position as Trump seeks leverage to force trading partners to halt the flow of migrants and drugs, but they underscored how much has changed since the election. Before Trump’s arrival on the political stage, presidents of both parties had spent decades bringing down trade barriers, but the president-elect has made clear he intends to build them back up again.

Daniel Price, who was an international economics adviser to President George W. Bush, said Trump appeared to be using the tariff threats to extract concessions.

“This isn’t a surprise, as it’s what he said on the campaign trail,” Price said. “He’s looking for offers of appeasement from his targets to mitigate any tariffs.”

Miriam Sapiro, an acting U.S. trade representative under President Barack Obama, said Trump had an incentive not to wait until his inauguration. Trump’s narrow 1.6-percentage- point election victory with little coattails suggests that Republicans are in danger of losing one or both houses in the midterm elections if history is a guide.

“It’s unusual, but not surprising, that Trump wants to move quickly because he can’t run again, and he’ll have only two years of real power if Republicans lose control of Congress in 2026,” said Sapiro, who is now at the Center for Strategic and International Studies in Washington. “But telegraphing early what he intends to do when he takes office gives other countries, as well as U.S. and foreign companies, more time to organize their moves.”

Trump is not confining his pre-inauguration international dealings to economics. He authorized his billionaire financial patron Elon Musk to open discussions with Iran, bypassing the current administration, which is laboring to manage a precarious standoff between the Islamic republic and Israel. Trump has vowed to end Russia’s war in Ukraine before the inauguration, a task that he says without embarrassment or explanation should take him a mere 24 hours when he gets around to it. By one account, he has even had a phone conversation with President Vladimir Putin of Russia, although the Kremlin denied it.

The Biden and Trump teams have had only glancing consultations since the election because the president-elect refused for weeks to sign memorandums of understanding on how to handle the transition with the federal government. In part because of that, Secretary of State Antony Blinken had spoken with Sen. Marco Rubio, the Florida Republican who is Trump’s pick for that post, but had not been able to sit down in person.

Amos Hochstein, Biden’s envoy negotiating Middle East peace, briefed Trump’s aides on his talks to end the fighting in Lebanon shortly after the Nov. 5 election and then again last week when an agreement was imminent. He came away convinced that the Trump team supported the Biden administration’s approach, if only because it would benefit Israel, according to a senior administration official who insisted on anonymity to discuss conversations between the two camps.