The outgoing NATO chief Jens Stoltenberg, in his final speech Thursday, will warn the United States against "isolationism" as Donald Trump seeks to return to the White House.
"We have heard voices on both sides of the Atlantic calling for America and Europe to part ways," he will say, according to excerpts of his speech seen by Agence France-Presse (AFP).
"Focusing on short-sighted national interests over long-term cooperation will not serve us well. Isolationism will not keep anyone safe."
The parting warning comes as Washington's allies wait to see if former President Trump will win the U.S. presidential election in November. They fear he would loosen Washington's commitment to NATO.
Trump has said the United States could stop protecting NATO members that do not spend enough on defense.
Stoltenberg is set to hand over the reins at the Western military alliance to former Dutch Prime Minister Mark Rutte on Oct. 1 after a decade at the helm.
During that time he has helped oversee a major increase in defense spending from European members spurred by pressure from Washington and, more importantly, Russia's war on Ukraine.
At the last count, 23 of NATO's 32 countries were set this year to reach the alliance's target of spending two percent of their gross domestic product on defense set back in 2014.
"The good news is that we have delivered on the pledge we made ten years ago," Stoltenberg will say.
"But the bad news is that this is no longer enough to keep us safe."
As Moscow's war on Ukraine grinds on through its third year, Stoltenberg will insist that NATO allies need to ensure Kyiv can negotiate "from a position of strength" when the time comes to talk.
"Any future peace deal must be backed by strong and sustained military support. Not just pieces of paper."
NATO's ties to Russia have been cut in the wake of the Kremlin's all-out invasion of Ukraine in 2022.
Stoltenberg will argue that at some point the alliance needs to return to dialogue with Moscow on issues such as arms control.
"We have to speak to our neighbors. However difficult it might be. But dialogue only works when it is backed by strong defences."