With Western powers increasingly concluding Ukraine has lost Crimea to Russia, the U.S. and its allies face few viable options and serious questions over future relations.
In ignoring President Barack Obama's Friday warning to keep out of Ukraine, Russia looks to be precipitating the greatest crisis in Russia-Western relations since at least the fall of the Berlin Wall.
How events play out in the next few days could help shape the geopolitical map for years to come.
Any Western direct military action would risk a war between nuclear superpowers. Ukraine's relatively small and underequipped forces could take action but would risk inciting a much wider Russian invasion that could overrun the country.
Obama in particular faces some domestic calls to support Ukraine, although appetite for military involvement appears almost entirely absent. On Saturday, the Pentagon said there had been no change to its military deployments.
"For the West, it's a very difficult position," said Nikolas Gvosdev, professor of national security at the US Naval War College. "Obama effectively set down the US red lines," he said.
Putin has gone right through them." Russian forces without official insignia have taken control of key facilities in Ukraine's Black Sea Crimean peninsula over the past three days and surrounded Ukrainian military units.
The best that can now be done, some current and former officials say, is to avoid a further escalation that sees Moscow take over industrialised eastern Ukraine - also mainly Russian-speaking and far larger and more economically significant.
Russian troops are engaged in war games near the border with Ukraine and pro-Russian activists have hoisted Russian flags at government buildings in the east, clashing with supporters of Ukraine's new authorities, but there has been no sign of Russian military action there so far.
Washington and other NATO powers must also find a way to reassure increasingly flustered Eastern European states -particularly the former Soviet Baltics - that their defence guarantees will be honoured, without escalating tensions.
The risk of missteps is high. As well as conventional forces, Russia could cut off gas supplies to Europe, which run through Ukraine, and is believed to have sophisticated cyber attack capabilities it could turn on Ukraine or the West.
"This is arguably the most dangerous situation in Europe since the Soviet invasion of Czechoslovakia in 1968," said one Western official on condition of anonymity. "With troops at high readiness on exercise in (Russia's) western military district they are in a strong position."
West Holds Back
Soviet troops invaded Czechoslovakia in 1968 after the "Prague Spring" saw a more moderate government come to power seen as much more open to the West.
Despite Czech calls for support, Washington and its allies offered little more than criticism, reluctant to risk nuclear war following the Cuban Missile crisis six years earlier.
The current stand-off is more dangerous than that over the 2008 Georgia war, where the West held back in part because the Georgian government was blamed for escalating the war through an attempt to seize the disputed region of South Ossetia.
In sending troops to Ukraine, in contrast, Moscow is seen to have unilaterally invaded a sovereign state - although there have long been Russian forces in Crimea, which leases the base for its Black Sea Fleet in Sebastopol from Ukraine.
NATO states have no legally binding alliance ties to Ukraine, although Western officials have been broadly supportive of those who ousted pro-Moscow President Viktor Yanukovich last week after dozens of pro-Europe protesters were shot dead.
Ukraine's borders were also guaranteed by the 1994 Budapest Memorandum, also signed by Russia, the US and Britain, in return for giving up Soviet-era nuclear weapons left in the country after the Soviet Union's collapse.
Last week, NATO's Supreme Allied Commander Europe U.S. General Philip Breedlove told reporters the alliance had no military plans to support Ukraine if attacked.
In an article for Foreign Policy magazine on Saturday, Breedlove's predecessor said that should quickly change.
"The hope is that cooler heads will prevail," retired Admiral James Stavridis wrote. "However, hope is not a strategy, and therefore further action should be considered. Planning is vital to laying out options to decision makers, and NATO's military planners should have a busy weekend at least."
NATO ambassadors held emergency talks in Brussels on Sunday with European foreign ministers to meet on Monday.
Ukraine participates in various NATO operations and has formed a consultative commission with the alliance. Officials say the commission may meet in the coming days and could request that NATO headquarters begins some contingency planning.
During the 2008 Georgia war, Washington sent warships into the Black Sea to deliver aid and diplomatic support. Two U.S. warships, the assault ship USS Mount Whitney and destroyer USS Taylor, were deployed earlier this month to provide security support for the Sochi Winter Olympics.
Sending them towards Ukraine could be seen as provocative, however. To make matters more complex, USS Taylor ran aground near a Turkish port on February 12 and was damaged.
"Realistically, we have to assume the Crimea is in Russian hands," the Western official said. "The challenge now is to deter Russia from taking over the Russian-speaking east of Ukraine."
For now, the West is falling back on political and economic measures, starting with several countries pulling out of preparatory meetings for June's Russia-hosted G8 summit and recalling their ambassadors from Moscow.
U.S. Secretary of State John Kerry threatened sanctions on Sunday, mentioning visa bans, asset freezes and trade isolation as possible steps.
At their most extreme, financial sanctions could target senior Russian officials - perhaps even Putin himself -- and in the longer term, Europe will try to wean itself off Russian gas.
NATO is seen almost certain to cancel a range of joint meetings with Moscow and pull out of joint anti-terror exercises. The alliance could also decide to extend membership -or lesser ties - to both Georgia and Ukraine, although that might prove several steps too far for some member states.
More major exercises and shows of force from NATO in areas bordering Russia now appear all but inevitable, building on November's "Steadfast Jazz" Baltic drills.
Ukraine's military, now ordered to full combat readiness to repel a full Russian invasion, is considerably weaker than Russia's. London-based think tank the International Institute for Strategic Studies (IISS) says it has some 129,950 military personnel. Russia mobilised up to 150,000 troops on Friday in its western military district in what it called a planned drill.
Ukrainian special forces or irregular units could mount hit-and-run attacks on Russian forces in the country. For now, however, they are seen holding back.
"My feeling is that if this remains just Crimea, the Ukrainians will let it go for now," says Dmitry Gorenburg, Russia analyst at the US government-funded Centre for Naval Analyses, part of the larger not-for-profit CNA Corporation.
"But if Russia looks like it's going to take the rest of eastern Ukraine, they will fight even if it means they know they will lose."
Some analysts explicitly compare events in Crimea with Nazi Germany's 1938 annexation of Czechoslovakia's German-speaking Sudetenland, followed months later by the rest of the country and the next year by Poland, sparking the Second World War.
The important thing now, they argue, is to make sure Russia understands which lines - such as those around NATO Baltic members - really cannot be crossed.
In Poland, Prime Minister Donald Tusk said the Ukrainian conflict could accelerate Warsaw's efforts to modernise the army and gain energy independence.
But most capitals, including Washington, have little economic choice but to cut defence spending.
"The Russian military still doesn't really compare to ours," said former U.S. Navy officer Christopher Harmer, now senior analyst at the Institute for the Study of War in Washington DC.
"But they know where they want to use it and unlike us, they have the will to do so."