The fractured relationship between President Barack Obama and Russian leader Vladimir Putin has reached a critical juncture in the conflict over Ukraine, with ties between the two nations more strained than at any time since the Cold War.

Russia’s annexation of Crimea last year and its support for separatists in Ukraine’s east have soured Obama on Putin and prompted U.S.-led sanctions that have helped push Russia’s economy toward recession.

The confrontation “will continue and could escalate pretty easily,” said Fiona Hill, a senior fellow at the Brookings Institution in Washington and author of “Mr. Putin: Operative in the Kremlin.”

The standoff between Obama and Putin complicates efforts to defuse the Ukrainian conflict. The U.S. also needs Russia’s help in the Iranian nuclear talks and in trying to thwart Islamic State in the Middle East.

It wasn’t long ago that Obama took a different view, beginning his presidency by offering Russia a “reset” and new era of cooperation. These days, he fulminates that Putin views the world through a “Cold War lens” of the past.

The relationship frayed almost as soon as Putin regained Russia’s presidency in 2012.

Initially, Obama and his aides saw the Russian leader’s return as little more than a routine transition. Instead, Putin’s fears of U.S. and European encroachment intensified with the toppling of a friendly government in Ukraine, a former White House aide and Russia scholars say.

“We’re in for a long and troubled relationship that’s going to take a lot of energy to manage,” said Michael McFaul, Obama’s one-time ambassador to Russia and an architect of the administration’s early outreach. “I do not believe it’s possible to pivot back into a second reset.”

The differences between Obama and Putin seem personal. Putin granted asylum to fugitive U.S. security contractor Edward Snowden and lectured Obama on the dangers of American exceptionalism in an opinion article published in The New York Times in September 2013.

The Obama administration didn’t appreciate the change in direction that was coming in 2011 when Putin, who had stepped back from the presidency to become prime minister, decided to run for the office again rather than back the re-election of his ally Dmitry Medvedev, who was nearing the end of his term.

“We underestimated how big a change that was,” said McFaul, now a political science professor at Stanford University. “I briefed the president with the view that there should be continuity since we assumed Putin played a central role in foreign policy.”

McFaul traces the undoing of the reset policy to Putin’s response to street demonstrations against the regime in December 2011 following allegations of fraud in parliamentary elections.

Putin saw a hidden U.S. strategy of “regime change” in the protests, as well as in the advice U.S. and European officials were privately giving to Russians against his campaign for another presidential term, the hostile Western media coverage, and the later ouster of Ukrainian President Viktor Yanukovych, according to Hill.

“He is convinced, completely convinced that we are a threat to Russia, that we overthrew Yanukovych, and that we would still like to overthrow him,” Hill said. “He’s trying to teach us a lesson.”