It's been building for months. And now, according to some, Russia has launched a "full-scale invasion" of Ukraine.
U.S. officials say Russian troops were directly involved in the latest fighting, alongside pro-Russian rebels.
"Russia is responsible for the violence in eastern Ukraine. The violence is encouraged by Russia. The separatists are trained by Russia; they are armed by Russia; they are funded by Russia," President Barack Obama told reporters Thursday.
Moscow, meanwhile, has said it would do everything possible to protect ethnic Russians in eastern Ukraine, but denies direct involvement in the fighting. It says the United States must stop interfering.
A Russian senator and the deputy head of the Committee on Defense and Security in Russia's upper house of Parliament, Evgeny Serebrennikov, dismissed reports of a Russian incursion as patently untrue.
"We've heard many statements from the government of Ukraine, which turned out to be a lie. What we can see now is just another lie," he told the Russian state news agency RIA Novosti.
As the stakes seem higher than ever in Ukraine, what does this latest development -- a significant one, most believe -- mean?
CNN.com breaks down some basics:
How did we get here?
Last December, then-President of Ukraine Viktor Yanukovych backed out of a trade agreement long in the works with the European Union and chose to take a $15 billion loan from Russia for his economically hurting nation. That move angered many Ukrainians who wanted their country to move more in line with the EU, and who also saw their president as a corrupt politician who'd done little to help the nation's limping economy, experts say. Protests broke out in the streets of Ukraine's capital, Kiev.
Shortly afterwards Yanukovych lost power, and Russian President Vladimir Putin ordered military exercises in Russia, just across Ukraine's border. Before February was over, armed men had seized regional parliament and government buildings in Crimea, a peninsula in southern Ukraine Russian whose population is mostly ethnic Russian.
In March, Russian troops annexed Crimea. At one point, a security camera at Ukrainian military base captured a bizarre sight -- a Russian armored personnel carrier busting through a base gate. Since then, fighting between Ukraine's military and pro-Russia rebels has continued to rage in eastern Ukraine.
Why does Russia care so much what happens in Ukraine?
There are strong cultural and historical ties between Ukraine and Russia. More than 100 years ago, Ukraine was part of imperial Russia.
In March, Putin gave a speech at the Kremlin in which he said Russia planned to "welcome back" Crimea.
"Crimea has always been an inseparable part of Russia," he said.
A New Yorker piece by journalist George Packer argues that Russia will "risk almost anything" to keep Crimea while the United States and Europe have minimal interests there and won't expend the risk to reverse the annexation.
A former ambassador to Ukraine explained in June to Congress why the United States should care about the nation, ranging from the fact that it's been a solid international partner on nuclear issues and in the war in Iraq.
Steven Pifer said that the "illegal seizure of Crimea is the most blatant land-grab that Europe has seen since 1945."
If Europe and the United States don't adequately respond, "the danger is that Mr. Putin may pursue other actions that would further threaten European security and stability," he said.
What has the West done, what can it do?
The West can always do something. But what is realistic and practical?
Most experts believe that answers are diplomacy and sanctions.
Charles Kupchan, a professor of international affairs at Georgetown University, says the United States and allies can keep trying to isolate Russia diplomatically -- prohibit Russian leaders from attending major summits or temporarily halt trade talks, for example.
"The dialogue should continue," Kupchan said, on a conference call initiated by the Council on Foreign Relations. "We have to look at this crisis with a certain amount of sobriety, in the sense that we still need Russian cooperation, if we can get it, on a lot of issues."