- People in Crimea, an autonomous region of Ukraine, vote Sunday on joining Russia
- The United States and its European allies reject the referendum as illegal
- To Russia, the ousting of the elected Ukraine government was illegal
- At issue is whether constitutional authority exists in Ukraine
To President Barack Obama and U.S. allies in Europe, Sunday's secession referendum in Crimea is unconstitutional, illegal and a fraud because Russian troops have essentially taken over the southern Ukraine peninsula.
To Russian President Vladimir Putin, it is a chance for Crimean residents to decide if they want to realign their region with Moscow after the political strife in Ukraine that ousted the pro-Russian leader last month.
Whatever the outcome, the vote planned by Crimea's regional parliament and endorsed by Russia's government will further inflame the Ukrainian crisis as the United States and European Union seek a diplomatic solution while threatening diplomatic and political sanctions.
Here are some of the biggest questions about the issue, with a look at how key players are weighing in:
1) What is the Crimean referendum?
Voters in the autonomous Ukrainian region of about 2 million people will choose between asserting independence from the former Soviet republic or joining neighboring Russia. There is no option for the status quo -- remaining a part of Ukraine.
Voters must be older than 18 and bring ID documents to show they are a resident of Crimea in order to cast a ballot. Officials say voters can expect to see members of the local electoral commission and observers at their allocated polling station, which will be open from 8 a.m. to 8 p.m. local time.
Results are expected Monday and U.S. officials have made clear they expect Crimean voters to choose to become part of Russia.
The referendum follows the lightning chain of events in Ukraine in recent months that included increasingly violent protests against the government, the country's pro-Russian President fleeing across the border, and then Russian troops seizing what amounts to military control of Crimea -- where Russia's Black Sea fleet is based.
By unanimously backing the referendum, the Crimean parliament signaled the intentions of regional leaders as well as Russia to restore the territory to Moscow's control.
Ukraine's interim government insists Crimea is part of the country, a stance supported by the United States and its European allies.
2) Is it legal?
Depends on who you ask.
In Ukraine, the interim government rejects the referendum as unconstitutional, while many in the pro-Russian Crimea region desire the chance to again align with Moscow.
Russia says the political upheaval in Ukraine that forced out President Viktor Yanukovych ended the Kiev government's constitutional authority.
Russian President Vladimir Putin calls the ousting of Yanukovych an anti-constitutional coup, and he argues the people of Crimea should have the right to decide their future in the same way as other autonomous regions, such as Kosovo's breakaway from Serbia.
However, Obama and European leaders insist the Crimean referendum would violate the Ukrainian constitution and international law.
Samantha Power, the U.S. ambassador to the United Nations, told CNN last week that the Ukrainian constitution requires territorial referendums to be held "across the entirety of Ukraine and not just in a sub-region like this."
She argued that Yanukovych fled of his own volition, and that constitutional authority agreed to in a February 21 deal he reached with opposition leaders and European envoys remained in effect.
Under that agreement, Ukraine will elect a new government on May 25 for the entire country, including Crimea.
3) What would it mean?
Obama's quick condemnation of a Crimean referendum showed the United States expects the outcome to favor realigning with Russia.
The autonomous region has a 60% ethnic Russian population, having been part of Russia until it was ceded to Ukraine in 1954 when both countries belonged to the Soviet Union.
Power said that if the referendum goes forward, the result would get little recognition beyond Russia.
That didn't matter in 2008 when Russia under Putin sent troops into neighboring Georgia, another former Soviet territory, to back autonomy for the pro-Russian regions of Abkhazia and South Ossetia.
More than five years later, both territories are effectively annexed with Russian forces still present.
Asked if Crimea and perhaps other parts of eastern and southern Ukraine that favor Russia might end up the same, Power said: "I think it's maybe not helpful to talk about precedents."
"We are seeking to create a future where the past is not replicated here in Ukraine," she added.
4) Potential fallout?
For Russia, a major motivation is ensuring control of its Black Sea fleet based in Sevastopol, as well as its economic influence in Ukraine and other former Soviet territories and satellites.
The Crimean referendum would provide an electoral justification for what amounts to a land grab amid Ukraine's political upheaval.
Obama acknowledged Russia had legitimate concerns that needed to be part of a solution, such as guarantees it could maintain the Sevastopol-based naval fleet in Ukraine.
However, his call for Russian troops in Crimea to return to their barracks and negotiations between Russia and Ukraine on a solution has so far yielded little progress.
Both the United States and European Union have threatened sanctions if no significant progress occurs soon, but major economic ties between Russia and European powers, such as Germany, raise questions about the strength of any EU steps.
Secretary of State John Kerry said Thursday that if the Crimean referendum proceeds with no sign of movement by Russia on negotiating with Ukraine on the crisis, "there will be a very serious series of steps Monday in Europe and here."
To CNN's Fareed Zakaria, Putin may get his wish with Crimea, but at a cost.
"They got Crimea, but they've scared off Ukraine," he said. "And, most importantly, the people of Ukraine are now deeply suspicious of Russia. The people in Poland and Hungary and the Czech Republic, or probably in places like Kazakhstan, are all now fearful."