But the message from the Barack Obama administration was clear: avoid military confrontation with Moscow.
The White House's message to Kiev was advice, not an order, U.S. and Ukrainian officials have recently told us, and was based on a variety of factors.
There was a lack of clarity about what Russia was really doing on the ground.
The Ukrainian military was in no shape to confront the Russian Spetsnaz (special operations) forces that were swarming on the Crimean peninsula.
Moreover, the Ukrainian government in Kiev was only an interim administration until the country would vote in elections a few months later.
Ukrainian officials told us that other European governments sent Kiev a similar message.
But the main concern was Russian President Vladimir Putin.
As U.S. officials told us recently, the White House feared that if the Ukrainian military fought in Crimea, it would give Putin justification to launch greater military intervention in Ukraine, using similar logic to what Moscow employed in 2008 when Putin invaded large parts of Georgia in response to a pre-emptive attack by the Tbilisi government.
Russian forces occupy two Georgian provinces to this day.
Looking back today, many experts and officials point to the decision not to stand and fight in Crimea as the beginning of a Ukraine policy based on the assumption that avoiding conflict with Moscow would temper Putin's aggression.
But that was a miscalculation.
Almost two years later, Crimea is all but forgotten, Russian-backed separatist forces are in control of two large Ukrainian provinces, and the shaky cease-fire between the two sides is in danger of collapsing.
"Part of the pattern we see in Russian behavior is to test and probe when not faced with pushback or opposition," said Damon Wilson, the vice president for programming at the Atlantic Council.
"Russia's ambitions grow when they are not initially challenged. The way Crimea played out, Putin had a policy of deniability, there could have been a chance for Russia to walk away."
When Russian special operations forces, military units and intelligence officers seized Crimea, it surprised the U.S. government.
Intelligence analysts had briefed Congress 24 hours before the stealth invasion, saying the Russian troop buildup on Ukraine's border was a bluff.
Ukraine's government -- pieced together after President Viktor Yanukovych fled Kiev for Russia following civil unrest -- was in a state of crisis.
The country was preparing for elections and its military was largely dilapidated and unprepared for war.
There was a debate inside the Kiev government as well.
Some argued the nation should scramble its forces to Crimea to respond.
As part of that process, the Ukrainian government asked Washington what military support the U.S. would provide.
Without quick and substantial American assistance, Ukrainians knew, a military operation to defend Crimea could not have had much chance for success.
"I don't think the Ukrainian military was well prepared to manage the significant challenge of the major Russian military and stealth incursion on its territory," said Andrew Weiss, a Russia expert and vice president for studies at the Carnegie Endowment, told us.
This was also the view of many in the U.S. military and intelligence community at the time.
There was also the Putin factor.
In the weeks and months before the Crimea operation, Russia's president was stirring up his own population about the threat Russian-speakers faced in Ukraine and other former Soviet Republics.
"They did face a trap," said the Atlantic Council's Wilson, who was the senior director for Europe at the National Security Council when Russia invaded Georgia in 2008.
"Any Ukrainian violent reaction to any of these unknown Russian speakers would have played into the narrative that Putin already created, that Ukraine's actions threaten Russian lives and he would have pretext to say he was sending Russian forces to save threatened Russians."
The White House declined to comment on any internal communications with the Ukrainian government.
A senior administration official told us that the U.S. does not recognize Russia's occupation and attempted annexation of Crimea, and pointed to a series of sanctions the U.S. and Europe have placed on Russia since the Ukraine crisis began.
"We remain committed to maintaining pressure on Russia to fulfill its commitments under the Minsk agreements and restore Ukraine's territorial integrity, including Crimea," the senior administration official said.
Ever since the annexation of Crimea in March, 2014, there have been a group of senior officials inside the administration who have been advocating unsuccessfully for Obama to approve lethal aid to the Ukrainian military.
These officials have reportedly included Secretary of State John Kerry, his top Europe official, Victoria Nuland, Defense Secretary Ashton Carter, and General Philip Breedlove, the supreme allied commander for NATO.
Obama has told lawmakers in private meetings that his decision not to arm the Ukrainians was in part due to a desire to avoid direct military confrontation with Russia, one Republican lawmaker who met with Obama on the subject told us.
The U.S. has pledged a significant amount of non-lethal aid to the Ukrainian military, but delivery of that aid has often been delayed.
Meanwhile, Russian direct military involvement in Eastern Ukraine has continued at a high level.
Even former Obama administration Russia officials acknowledge that Ukraine's decision last year to cede Crimea to Moscow, while making sense at the time, has also resulted in more aggression by Putin.
"Would a devastating defeat in Crimea serve the interest of the interim government? Probably not," said Michael McFaul, who served as ambassador to Russia under Obama and is now a scholar at Stanford University's Hoover Institution.
But nonetheless, McFaul said, the ease with which Putin was able to take Crimea likely influenced his decision to expand Russia's campaign in eastern Ukraine:
"I think Putin was surprised at how easy Crimea went and therefore when somebody said let's see what else we can do, he decided to gamble.”
The Obama administration, led on this issue by Kerry, is still pursuing a reboot of U.S.-Russia relations.
After a long period of coolness, Kerry's visit to Putin in Sochi in May was the start of a broad effort to seek U.S.-Russian cooperation on a range of issues including the Syrian civil war.
For the White House, the Ukraine crisis is one problem in a broader strategic relationship between two world powers.
But for the Ukrainians, Russia's continued military intervention in their country is an existential issue, and they are pleading for more help.
While many Ukrainians agreed in early 2014 that fighting back against Russia was too risky, that calculation has now changed.
The Ukrainian military is fighting Russian forces elsewhere, and Putin is again using the threat of further intervention to scare off more support from the West.
If help doesn't come, Putin may conclude he won't pay a price for meddling even further.