WASHINGTON — The war in Ukraine has prompted the biggest rethinking of American foreign policy since the Sept. 11, 2001, attacks, infusing the United States with a new sense of mission and changing its strategic calculus with allies and adversaries alike.

The Russian invasion has bonded America to Europe more tightly than at any time since the Cold War and deepened U.S. ties with Asian allies, while forcing a reassessment of rivals like China, Iran and Venezuela.

And it has re-energized Washington’s leadership role in the democratic world just months after the chaotic U.S. withdrawal from Afghanistan ended 20 years of conflict on a dismal note.

But the new focus on Russia will come with hard choices and internal contradictions, similar to ones that defined U.S. diplomacy during the Cold War, when America sometimes overlooked human rights abuses and propped up dictators in the name of the struggle against communism.

“It feels like we’re definitively in a new era,” said Benjamin J. Rhodes, a former deputy national security adviser in the Obama White House. “The post-9/11 war on terror period of American hubris, and decline, is now behind us. And we’re not sure what’s next.”

The attack by President Vladimir V. Putin of Russia on his neighbor has become a prism through which nearly all American foreign policy decisions will be cast for the foreseeable future, experts and officials said.

In recent weeks, Western officials have spoken in terms that often echo the grand declarations that followed the 2001 terrorist attacks. On Friday, President Biden said that “the free world is coming together” to stand up to Mr. Putin — a phrase reminiscent of President George W. Bush’s talk of how “the entire free world” was at war against terrorism.

In the near term, Russia’s aggression is sure to invigorate Mr. Biden’s global fight for democracy against autocracies like Moscow, making vivid the threats to fledgling democracies like Ukraine. Yet three increasingly authoritarian NATO nations — Poland, Hungary and Turkey — play key roles in the coalition aiding Kyiv. And the United States is grappling with internal assaults to its own democracy.

The war lends urgency to Mr. Biden’s climate change agenda, reinforcing the need for more reliance on renewable clean energy over the fossil fuels that fill Russian coffers. Yet it has already generated new pressure to increase the short-term supply of oil from the likes of Venezuela’s isolated dictatorship and Saudi Arabia’s authoritarian monarchy.

And it creates a powerful new incentive for the United States to find ways of prying President Xi Jinping of China away from Mr. Putin, who is likely counting on diplomatic and economic lifelines from Mr. Xi amid crushing Western sanctions. But some administration officials see China as a lost cause and prefer to treat China and Russia as committed partners, hoping that might galvanize policies among Asian and European allies to contain them both.

While some experts warn that a renewed focus on Europe will inevitably divert attention from Asia, several top White House officials say the United States can capitalize on how the war has convinced some Asian governments that they need to work more closely with the West to build up a global ideological front to defend democracy.

“What we are seeing now is an unprecedented level of Asian interest and focus,” Kurt M. Campbell, the top White House official on Asia policy, said at a talk hosted by the German Marshall Fund of the United States.

“And I believe one of the outcomes of this tragedy will be a kind of new thinking around how to solidify institutional connections beyond what we’ve already seen between Europe and the Pacific,” he said.

America’s approach to the world was already undergoing a major shift, with the U.S. wars in Afghanistan and Iraq concluded, and conversations over Islamist terrorism no longer at the fore. Many war-weary Americans welcomed calls for a reduced military footprint overseas by President Donald J. Trump, who questioned NATO’s relevance and even flirted with withdrawing from the alliance.

Mr. Biden sought to rebuild American alliances, but did so largely in the name of confronting China. The Russian invasion has expanded his mission dramatically and urgently, setting the stage for a seismic geopolitical shift that would pit the United States and its allies against China and Russia at once if they form an entrenched anti-Western bloc.

But it also gives Washington a new and nobler sense of purpose, Mr. Rhodes said. “We’ve been trying to get to a new era for a long time,” he said. “And now I think Putin’s invasion has necessitated an American return to the moral high ground.”

Playing Hardball Over Energy

Early signs of how the new American priorities are creating diplomatic quakes have already emerged.

On Friday, the United States and its European allies agreed to pause talks with Iran that just days earlier seemed on the verge of clinching a return to the 2015 deal that limited Iran’s nuclear program. Western nations are refusing a demand by Moscow, which is a party to the Obama-era agreement from which Mr. Trump withdrew, for guarantees that its future transactions with Iran be exempted from the sanctions imposed on Russia in recent weeks.

“It’s been clear since last weekend that negotiations to revive the Iran deal could not be walled off from the Ukraine war,” Dalia Dassa Kaye, an Iran expert at the RAND Corporation, said on Friday.

Last year, Mr. Biden made a new agreement a core goal of his foreign policy. It is unclear whether one can be struck without Russia, which is a member of the commission that both supervises compliance with the deal and would take control of Iran’s excess enriched uranium.

The United States is also looking at Venezuela from a new angle. Senior Biden administration officials traveled to Venezuela two weeks after the Russian invasion, becoming the first to visit the country in years. Venezuela, a partner of Russia, is under heavy U.S. sanctions imposed years ago to weaken the repressive government of President Nicolás Maduro. In 2019, the Trump administration imposed additional sanctions on the state oil company, central bank and senior officials to pressure Mr. Maduro to step down.

Now, with Mr. Biden looking to increase global oil supplies to bring down prices, U.S. officials are talking to Mr. Maduro’s government about buying his oil again. The idea has drawn some sharp criticism in Congress, however, where Senator Bob Menendez, Democrat of New Jersey and the chairman of the Senate Foreign Relations Committee, fumed that “efforts to unify the entire world against a murderous tyrant in Moscow should not be undercut by propping up a dictator under investigation for crimes against humanity in Caracas.”

The same imperative on oil is reshaping U.S. diplomacy with Saudi Arabia and the United Arab Emirates, two Persian Gulf nations that some Biden administration officials view with suspicion or hostility because of their autocratic systems and leading roles in a war in Yemen that has resulted in a humanitarian catastrophe. Brett McGurk and Amos J. Hochstein, two senior administration officials, traveled to the Gulf days before the Russian invasion to discuss security and energy issues.

However, Saudi Arabia has declined so far to increase oil production, while the United Arab Emirates waited until Wednesday to ask the OPEC nations to do so. American officials were also furious with the U.A.E. for declining to vote on a United Nations Security Council resolution to condemn Russia, though it did support a similar resolution later in the U.N. General Assembly.

The unreliability of the two nations and Russia’s place in the oil economy have increased momentum within the Biden administration to enact policies that would help the United States more quickly wean itself off fossil fuels and confront the climate crisis. This could lead future administrations to devote fewer diplomatic and military resources to the Gulf nations in the long term, even if U.S. officials want them to help on oil now.

“We may see more fundamental questioning about the value of these partnerships,” Ms. Kaye said. “These states already believe the U.S. has checked out of the region, but their stance on Russia may only strengthen voices calling for a further reduction of U.S. forces in the region.”

Israel, the closest U.S. ally in the Middle East, has also staked out a neutral position on the Ukraine war, largely because of Russia’s presence in the region. But American officials have been more forgiving of Israel’s stance as Prime Minister Naftali Bennett conducts shuttle diplomacy. He met with Mr. Putin for three hours in Moscow on March 5 and then spoke with Volodymyr Zelensky, the president of Ukraine, by phone before returning home. U.S. officials say Mr. Bennett consulted with them about the talks, and Secretary of State Antony J. Blinken said this past week that they “appreciate the efforts.”

Juggling Allies in Europe and Asia

In Europe, Russia’s invasion has supercharged the Biden administration’s efforts to restore the morale of a NATO alliance that Mr. Trump undermined.

But the alliance includes three nations — Poland, Hungary and Turkey — whose democratic backsliding has troubled the Biden administration. Hungary and Turkey were pointedly excluded from Mr. Biden’s global democracy summit in December, and the European Union has cut billions of euros of funding to Poland and Hungary for what it sees as erosions of legal and democratic principles. Now all three countries are participating in the coalition against Russia.

“In times of crisis, there is sometimes a tension between our values and our interests,” said Andrea Kendall-Taylor, a senior fellow at the Center for a New American Security. “In the short term, we’re going to have to prioritize pushing back against Russia, at the risk of taking our foot off the gas on the democracy and human rights concerns that had been at the front and center of the Biden administration’s agenda.”

In Asia-Pacific region, several important U.S. partners and allies are working with Washington on sanctions and export controls on technology against Russia. These include Japan, South Korea, Taiwan, Singapore and Australia. Some Asian nations have agreed to long-term gas swaps with Europe to help relieve a potential Russian shut-off of energy exports. And Australia has committed to spending $50 million to send weapons to Ukraine, including missiles and ammunition.

However, India — the most populous U.S. ally in the so-called Quad coalition of democracies in Asia — has refrained from condemning Russia’s invasion because of decades-old security ties with Moscow. That stance undermines Mr. Biden’s insistence that democratic nations band together against autocracies.

But it is the other Asian behemoth, China, that presents the biggest diplomatic challenge for the United States. China is Russia’s most powerful partner, and their bond has strengthened in recent years.

Even as the Russian military decimates Ukrainian cities and kills hundreds or thousands of civilians, China has signaled that it stands by Moscow by issuing anti-U.S. declarations and amplifying the Kremlin’s propaganda and conspiracy theories.

Mr. Xi’s persistent support of Mr. Putin, with whom he shares a drive to dilute American power, has made administration officials wonder whether there is any way to pull them apart on Ukraine.

On Thursday, William J. Burns, the C.I.A. director, told U.S. senators he believed that Mr. Xi was “unsettled” by the war. Some China analysts say that if Beijing wants to salvage its reputation with Western nations, particularly in Europe, it might agree to take steps to help Ukraine without directly breaking from Russia.

Ryan Hass, a China director on the National Security Council in the Obama White House, proposed testing Beijing with specific requests, such as asking them to provide more humanitarian aid and refrain from recognizing Russian-installed governments in Ukraine or shielding Russia from war crimes investigations.

“If China’s leaders take concrete actions to relieve suffering,” he said, “then lives would be saved and there would be less centrifugal pressure toward cleaving the world into rival blocs.”