MUNICH — Chancellor Angela Merkel of Germany delivered a strong rejoinder on Saturday to American demands that European allies pull out of the Iran nuclear deal and gave a spirited defense of multilateral institutions in a world increasingly marked by great-power rivalry.

In an uncharacteristically passionate speech, Ms. Merkel said the nuclear deal was the best way of influencing Iranian behavior on a range of non-nuclear issues, from missile development to terrorism.

Without mentioning President Trump or the United States by name in what may be her last speech to this major security conference, Ms. Merkel criticized other unilateral moves, such as Mr. Trump’s decision to pull American troops out of Syria, a suggestion that he would withdraw quickly from Afghanistan and his decision to suspend the Intermediate Range Missile Treaty with Russia, which directly affects European security.

“We sit there in the middle with the result,” she said.

Ms. Merkel spoke immediately before the United States vice president, Mike Pence, and addressed a packed auditorium with an audience that included Mr. Trump’s daughter Ivanka, as well as the Russian foreign minister and a high-ranking Chinese official, who all pointedly remained seated when the chancellor received a standing ovation.