"Biden just finished his surrender ('speech') at the United Nations, and nobody, despite all we give them, showed up. No respect for America any longer!" Trump posted.
Indeed, French President Emmanuel Macron and U.K. Prime Minister Rishi Sunak skipped the assembly, leaving the U.S. as the sole member out of five permanent members of the U.N. Security Council to attend.
"While Sunak and Macron have an excuse, I do think it is telling that they are absent," Richard Gowan, U.N. director for the International Crisis Group, told CBS News.
India's Prime Minister Narendra Modi also did not attend.
Chinese President Xi Jinping also skipped, as he did last week's G-20 in India. Russian President Vladimir Putin didn't attend either.
In his speech, Biden appealed to the world leaders who did attend — including Ukrainian President Volodymyr Zelenskyy and the Russian ambassador to the U.N. — to stand with Ukraine against Russia's invasion.
"Russia believes that the world will grow weary and allow it to brutalize Ukraine without consequence," Biden said. "If we allow Ukraine to be carved up, is the independence of any nation secure?"
The Biden administration is working with Congress to provide another $24 billion to bolster Ukraine. Trump has said he will not commit to funding for Ukraine if he's elected in 2024, adding that he would end the war in 24 hours.
Zelenskyy, who gave a speech later Tuesday, will visit the White House on Thursday and is expected to meet with congressional leaders.
Even as he takes flak from Trump over perceived weakness in his U.N. address, Biden is under fire over yet another element of his foreign policy: a prisoner swap deal recently struck with Iran.
Indeed Republicans including Trump have expressed outrage over the deal, which comes even as Iran has seemed to defy other nations in advancing a nuclear weapons program.
The administration notified Congress last week that it was proceeding with the agreement, which involved issuing a waiver that will let Tehran tap into some $6 billion in oil revenue that the U.S. had frozen through sanctions.