For much of his time in office, President Barack Obama has been accused by a mix of conservative hawks and liberal interventionists of overseeing a dangerous retreat from the world at a time when American influence is needed most.

The once-hopeful Arab Spring has staggered into civil war and military coup. China is stepping up territorial claims in the waters off East Asia. Longtime allies in Europe and in the Persian Gulf are worried by the inconsistency of a president who came to office promising the end of America's post-Sept. 11 wars.

Now Ukraine has emerged as a test of Obama's argument that, far from weakening American power, he has enhanced it through smarter diplomacy, stronger alliances and a realism untainted by the ideology that guided his predecessor.

It will be a hard argument, analysts say, for him to make.

A president who has made clear to the American public that the "tide of war is receding" has also made clear to foreign leaders, including opportunists in Russia, that he has no appetite for a new one. What's left is a vacuum once filled, at least in part, by the possibility of American force.

"If you are effectively taking the stick option off the table, then what are you left with?" said Andrew Kuchins, who heads the Russia and Eurasia Program at the Center for Strategic and International Studies. "I don't think that Obama and his people really understand how others in the world are viewing his policies."

Rarely has a threat from a U.S. president been dismissed as quickly - and comprehensively - as Obama's Friday night warning to Russian President Vladimir Putin. The former Cold Warrior and the former community organizer share the barest of common interests, and their relationship has been defined far more by the vastly different ways they see everything from gay rights to history's legacy.

From a White House podium, Obama told the Russian government late Friday that "there will be costs" for any military foray into Ukraine, including the semiautonomous region of Crimea, a strategically important peninsula on the Black Sea.

Within hours, Putin asked the Russian parliament for approval to send forces into Ukraine. The vote endorsing his request was unanimous, Obama's warning drowned out by lawmakers' rousing rendition of Russia's national anthem at the end of the session. Russian troops now control the Crimean Peninsula.

There are rarely good - or obvious - options in such a crisis. But the position Obama is in now, confronting a brazenly defiant Russia and few ways to meaningfully enforce his threat, has been years in the making. It is the product of his record in office and of the way he understands the period in which he is governing, at home and abroad.

At the core of his dilemma is the question that has arisen in White House debates over the Afghan withdrawal, the intervention in Libya, and the conflict in Syria: How to end more than a dozen years of American war and maintain a credible military threat to protect U.S. interests?

The signal Obama has sent - popular among his domestic political base, unsettling at times to U.S. allies - has been one of deep reluctance to use the heavily burdened American military even when doing so would meet the criteria he has laid out. He did so most notably in the aftermath of the U.S.-led intervention in Libya nearly three years ago.

But Obama's rejection of U.S. military involvement in Syria's civil war, where 140,000 people have died since he first called on President Bashar Assad to step down, is the leading example of his second term. So, too, is the Pentagon budget proposal outlined this past week that will cut the size of the army down to pre-2001 levels.

Inside the West Wing, there are two certainties that color any debate over intervention: that the country is exhausted by war and that the end of the longest of its post-9/11 conflicts is less than a year away. Together they present a high bar for the use of military force.

Ukraine has challenged administration officials - and Obama's assessment of the world - again.

At a North American summit meeting in Mexico last month, Obama said, "Our approach as the United States is not to see these as some Cold War chessboard in which we're in competition with Russia."

But Putin's quick move to war footing suggest a different view - one in which, particularly in Russia's backyard, the Cold War rivalry Putin was raised on is thriving.

The Russian president has made restoring Russia's international prestige the overarching goal of his foreign policy, and he has embraced military force as the means to do so.

As Russia's prime minister in the late summer of 2008, he was considered the chief proponent of Russia's military advance into Georgia, another former Soviet republic with a segment of the population nostalgic for Russian rule.

Obama, by contrast, made clear that a new emphasis on American values, after the excesses of the Bush administration, would be his approach to rehabilitating U.S. stature overseas.

Those two outlooks have clashed repeatedly - in big and small ways - over the years.

Obama took office with a different Russian as president, Dmitry Medvedev, Putin's choice to succeed him in 2008.

Medvedev, like Obama, was a lawyer by training, and also like Obama, he did not believe the Cold War rivalry between the two countries should define today's relationship.

The Obama administration began the "reset" with Russia - a policy that, in essence, sought to emphasize such areas as nuclear nonproliferation, counterterrorism, trade and Iran's nuclear program as shared interests worth cooperation.

But despite some successes, including a new arms-control treaty, the reset never quite reduced the rivalry. When Putin returned to office in 2012, so too did an outlook fundamentally at odds with Obama's.

Just months after his election, Putin declined to attend the Group of Eight meeting at Camp David, serving an early public warning to Obama that partnership was not a top priority.

At a G-8 meeting the following year in Northern Ireland, Obama and Putin met and made no headway toward resolving differences over Assad's leadership of Syria. The two exchanged an awkward back-and-forth over Putin's passion for martial arts before the Russian leader summed up the meeting: "Our opinions do not coincide," he said.

A few months later, Putin granted asylum to Edward Snowden, the former National Security Agency contractor whose disclosure of the country's vast eavesdropping program severely complicated U.S. diplomacy. Obama had asked for Snowden's return.

In response, Obama canceled a scheduled meeting in Moscow with Putin after the Group of 20 meeting in St. Petersburg last summer. The two met instead on the summit's sidelines, again failing to resolve differences over Syria.

It was Obama's threat of a military strike, after the Syrian government's second chemical attack crossed what Obama had called a "red line," that prompted Putin to pressure Assad into concessions. The result was an agreement to destroy Syria's chemical weapons arsenal, a process that is proceeding haltingly.

Since then, though, the relationship has again foundered on issues that expose the vastly different ways the two leaders see the world and their own political interests.

Russia's legislature passed anti-gay legislation and Obama included openly gay former athletes in the U.S. delegation to the Sochi Games.

New barbarities in Syria's civil war - and the near-collapse of a nascent peace process - have drawn sharper criticism from U.S. officials of Putin, who is continuing to arm Assad's forces.

How Obama intends to prevent a Putin military push into Ukraine is complicated by the fact that, whatever action he takes, he does not want to jeopardize Russian cooperation on rolling back Iran's nuclear program or completing the destruction of Syria's chemical weapons arsenal.

Economic sanctions are a possibility. But that decision is largely in the hands of the European Union, given that its economic ties to Russia, particularly as a source of energy, are far greater than those of the United States.

The most immediate threat that has surfaced so far: Obama could skip the G-8 meeting scheduled for June in Sochi, a day's drive from Crimea.

"If you want to take a symbolic step and deploy U.S. Navy ships closer to Crimea that would, I think, make a difference in Russia's calculations," Kuchins said. "The problem with that is are we really credible? Would we really risk a military conflict with Russia over Crimea-Ukraine? That's the fundamental question in Washington and in Brussels we need to be asking ourselves."