Why did Russian President Vladimir Putin risk political isolation, sanctions and other US-led retaliatory measures with a surprise incursion into neighbouring Ukraine? Because he can.
Putin's audacity is underpinned by the reality that, while patience is wearing thin, the Obama administration holds few options for punishing a leader who has already calculated that the fallout is worth the message to the world that Russia will fight fiercely for its interests.
Foreign policy analysts say the Ukraine invasion deals a severe blow to US-Russian ties, which already were fraying as it became clear that the Kremlin wasn't on board for what the Obama administration had envisioned as a "reset" in relations.
Putin, analysts say, doesn't seem concerned about a looming break-up with Washington - he is confident there will be no US military intervention and has decided to weather any other potential consequences.
Olga Oliker, a Russia specialist and senior international policy analyst at the RAND Corporation research centre in Washington, said the tug of war over Ukraine was part of a bigger divergence: competing visions from Washington and Moscow over the ideal future for Russia.
"We think what's good for Russia is stability in the neighbourhood, economic growth," Oliker said. "But Russia, or at least Vladimir Putin, thinks what's most important is that Russia is taken seriously, and that a strong Russia is one that sticks to its guns and gets what it wants."
In the case of the Ukraine, analysts say, it's likely that Putin will do just that. The United States and its allies could impose economic sanctions, kick Russia out of the G8 forum for industrialised democracies and issue stern condemnations, but none of that is likely to sway Putin.
By his calculation, the US and its allies need Russia more than Russia needs them. Russia exports natural gas to Europe, is the world's third-largest military spender, and wields veto power on the UN Security Council. As an ally of President Bashar al-Assad, Russia has made itself instrumental in talks on how to resolve the Syrian civil war.
Russia is also a key player in the effort to dismantle Syria's chemical weapons arsenal under an agreement that saved the Obama administration from making good on threats of a military strike and one of the six nations negotiating with Iran over its nuclear programme.
Obama spoke with Putin for 90 minutes on Saturday, asking Russia to withdraw its forces from Crimea and to initiate dialogue with Ukrainian officials. He also said that the US and its allies were meeting urgently to discuss the crisis - a suggestion of future punitive action - but the most serious threat, according to a White House statement, was that Russia could face "greater political and economic isolation".
"Obama has no particularly useful credibility to move the Russians," Steven Bucci, director of foreign and national security policy at the Heritage Foundation, a conservative research institute in Washington, said.
"The lack of decisive action in the past, the failure of the so-called 'reset' and the debt owed to Putin for rescuing Obama's dysfunctional Syria policy now gives little or no moral or diplomatic leverage. We are likely to be an impotent bystander in the tragedy that could be unfolding."
Several US lawmakers - across party lines - are incensed at Putin's conduct in Ukraine, viewing it as the most audacious in a series of stunts designed to embarrass the United States and undermine American interests.
Republican Senator John McCain, who has described Putin as "an old KGB colonel apparatchik that dreams of the days of the Russian Empire", said it was time to be realistic about "what President Putin is capable of doing in Ukraine". McCain called for unspecified "consequences" for the invasion.
In previous flare-ups with Russia, analysts say, US criticism was muted because the Obama administration was still trying to court Putin's help with Syria, Iran and other thorny diplomatic issues. The State Department was fond of reminding Moscow that the US had returned "many hundreds" of suspected criminals to Russia, and that the two governments had worked closely in the Boston marathon bombing investigation.
Such cajoling hasn't worked, it seems, but the Obama administration has been reluctant to give up on Russia and even now seems to be contemplating reprisal moves only reluctantly - and with the knowledge that any retaliation isn't likely to deter Putin.
"We're going to have to démarche strongly and suck it up," said Oliker, using the term for the equivalent of a diplomatic complaint. "Putin wouldn't have done this if he wasn't willing to pay."