WASHINGTON — A month ago, most Americans could not have found Crimea on a map. But its lightning-quick takeover by Moscow has abruptly redrawn the geopolitical atlas and may have decisively ended a 25-year period of often tumultuous but also constructive relations between the United States and Russia.

Since the fall of the Berlin Wall in 1989, Washington and Moscow had struggled to replace their Cold War rivalry with a new form of partnership, one that was tested by crisis after crisis but that endured in its own peculiar way. After each rupture, whether over Kosovo or Iraq or Georgia, came another reset that put the two powers back onto an uneasy equilibrium.

The decision by President Vladimir Putin to snatch Crimea away from Ukraine, ratified in a defiant treaty-signing ceremony in the Kremlin on Tuesday, threatens to usher in a new, more dangerous era. If it is not the renewed Cold War that some fear, it seems likely to involve a prolonged period of confrontation and alienation that will be hard to overcome. The next reset, if there ever is one, for the moment appears far off and far-fetched.

“This is an earthquake, and not a 4-point earthquake,” said Toby Gati, a longtime Russia specialist who served in President Clinton’s State Department and now works for the law firm Akin Gump on business deals.

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While it is not a return to the Cold War, she said, it does dispel the dreams of 1989.

“Europe whole and free? Well, it’s a Europe free-for-all,” Gati said. “And we don’t know how to react to it. And we don’t know how to control the narrative anymore.”

Stephen Hadley, who was President George W. Bush’s national-security adviser, said it would be harder to recover from this clash than in the past because Putin is effectively rejecting the international order established after the collapse of the Soviet Union.

“He wants to rewrite the history that emerged at the end of the Cold War,” Hadley said. “We have fundamentally different approaches to what Europe is going to be.”

At the White House on Tuesday, President Obama plotted his next moves, a tit-for-tat response of additional sanctions to punish Russia for what Vice President Joe Biden, speaking in Warsaw, described as “nothing more than a land grab.” In private moments, administration officials recognize that the chances of prying Crimea loose from Russia are minimal and that the real question is whether the West can stop Putin from destabilizing or even trying to take control of eastern Ukraine.

Even if the United States and Europe can draw that line, it is hard to see the relationship returning to business as usual in the short term. The steady integration of Russia into the international community, culminating with its entry into the World Trade Organization in 2012 with Obama’s help, headed into reverse Tuesday as the United States and six other industrial powers decided to meet next week as the Group of 7, effectively ending the Group of 8 that Russia joined in 1998.

Secretary of State John Kerry said Putin’s speech announcing the annexation “just didn’t jibe with reality.” Even though Russia has legitimate interests in Ukraine, he told a town-hall audience, “that doesn’t legitimize just taking what you want because you want it or because you’re angry about the end of the Cold War or the end of the Soviet Union, or whatever it is.”

In jeopardy are all sorts of areas where the United States and Russia cooperate. The two collaborate closely on space exploration, and U.S. access to the international space station depends entirely on Russian rocket launches. U.S. troops heading to and from Afghanistan fly through Russian airspace. Intelligence agencies share information about terrorist organizations, albeit not always everything. U.S. experts help Russians dismantle old nuclear weapons.

Even as Obama and Putin have traded diplomatic jabs, their governments have labored to limit the damage. Diplomats from the United States and Russia were in touch last week to ensure they could still work together on negotiations with Iran over its nuclear program, and then they returned to Geneva this week for renewed talks. After initially threatening to bar U.S. nuclear inspectors under the New START treaty Obama signed, Russian officials quietly told their counterparts the inspections would continue.

“We’ve been able to cooperate even as we have some differences, and serious differences, on other things,” Kerry said. “That’s the tragedy of what has happened with respect to Crimea.”

Whether that cooperation can continue beneath the radar if the West exacts the sorts of punishing sanctions it has been threatening seems more problematic. The relationship did resume after Russia’s war with Georgia in 2008. In that case, a newly inaugurated Obama made restoring ties a signature priority, and many in the West were willing to move on, in part because they blamed Georgia’s mercurial president, Mikhail Saakashvili, for provoking Moscow.

But long before Russian troops occupied Crimea, the relationship had spiraled downward, particularly since Putin formally resumed the presidency in 2012. He and Obama had little respect for each other, and Putin blamed the United States for street protests in Moscow. He brushed off Obama’s attempts to restart nuclear arms-reduction talks and gave shelter to Edward Snowden, the former National Security Agency contractor and leaker.

In response, Obama declared a “pause” in the relationship and canceled a trip to Moscow, the first time a president had scrubbed a Russian-U.S. summit meeting in more than a half-century.

By the time pro-Western street protests in Ukraine led to the ouster of its pro-Moscow president and Russia responded by sending troops to Crimea, a corner had been turned.

Michael McFaul, who was the optimistic and energetic architect of Obama’s Russia policy and later ambassador to Moscow, captured the sense of defeat in a mournful Facebook posting a few days ago.

“I am very depressed today,” he wrote, noting the coming isolation of Russia. “My only hope is that this dark period will not last as long as the last Cold War.”

Specialists said this would not be another Cold War, which was a global contest of ideology, pitting capitalism versus communism. Putin positions himself as leader of anti-American sentiment, but it is rooted in Russian nationalism rather than Marxist philosophy, and his main focus is on his own neighborhood.

But that does not mean it will not be challenging.

“It could turn into an extremely nasty and prolonged East-West conflict,” said Michael Dobbs, a Cold War historian and author who as a journalist covered the end of the Soviet Union. “It won’t be exactly like the Cold War because it won’t be a struggle for control of the world. But it will be something like Yugoslavia on a much larger scale and a more dangerous scale.”