"Only an ally can wish another ally to get stronger. This is not what an opponent of Europe would say," Tusk argued.

Since his first term, Trump has threatened to pull the U.S. out of the military alliance, insisting European countries chip in more money on defense rather than rely on the U.S. to pick up the slack on defense spending.

Three years into Russia's full-scale invasion of Ukraine, Ukrainian President Volodymyr Zelenskyy on Tuesday in Davos berated Europe for its lackluster defense spending as the war on its doorstep rages on indefinitely. The U.S. is the No. 1 donor of civilian and military aid to Ukraine, although Europe, in total summation, gives more.

Tusk said Poland, which has become NATO's and the European Union's front line since Russia's invasion in 2022, has upped its own defense spending closer to the 5 percent target.

Some EU countries, namely Germany, have also rejected joint borrowing for defense. For countries reticent to boost defense cash and oppose joint borrowing via eurobonds, Tusk said, “I'll be honest with you, it's not so important which method we are going to opt for to finance pan-European defense projects. "There is something which is more important and we should say it out loud: There is no alternative, there is no choice.”

He called repeatedly on Europe take charge of its own security while invoking former U.S. President John F. Kennedy.

"Don't ask America what it can do for our security. Ask yourselves what we can do for our own security," Tusk advised lawmakers.