President Biden outlined a sweeping vision of restored American global leadership on Thursday, announcing an end to U.S. support for Saudi Arabia’s military campaign in Yemen and vowing to confront Russia and China.

He also promised to work with allies to combat issues like the pandemic and climate change and announced a freeze on President Donald J. Trump’s planned troop redeployments from Germany.

After two weeks of emphasis on domestic issues, Mr. Biden visited the State Department to turn his focus to foreign policy and make good on campaign promises to revitalize American diplomacy, alliances and moral authority. Speaking to diplomats at the Harry S. Truman Building in Washington, Mr. Biden said he intended to “send a clear message to the world: America is back.”

“We’re going to rebuild our alliances,” Mr. Biden said. “We’re going to re-engage the world.”

In the first foreign policy speech of his administration, the president said he would rebuild “the muscle of democratic alliances that have atrophied over the past few years of neglect and, I would argue, abuse.”

In ending American support for offensive Saudi operations in Yemen’s civil war — which he said had “created a humanitarian and strategic catastrophe” — Mr. Biden is delivering on a campaign promise, days after his administration announced a review of major U.S. arms sales to Riyadh that were approved by the Trump administration. The U.S. has also provided intelligence, targeting data and logistical support for the Saudi intervention. Mr. Biden said he would work to revive dormant peace talks and announced the appointment of a special envoy for Yemen.

Mr. Biden announced he was “stopping any planned troop withdrawals from Germany,” halting Mr. Trump’s order to redeploy roughly 12,000 U.S. troops stationed in the country. National security experts from both parties had called Mr. Trump’s order shortsighted.