The United States and Britain “reaffirmed” their commitment to protect Ukraine’s borders in exchange for the nation giving up its nuclear weapons in a little-known agreement known as the “Budapest Memorandum signed by former President Bill Clinton in 1994.

The Daily Mail notes reports “if Russia has invaded Ukraine then it would be difficult for the US and Britain to avoid going to war.”

Sir Tony Brenton, who served as a British ambassador from 2004 to 2008, said war is certainly on the table if it’s determined that the Budapest Memorandum is “legally binding.”

According to the Daily Mail, Kiev has asked that the agreement be honored as it claims its borders have been violated.

Signing of the Budapest Memorandum (AP)

Signing of the Budapest Memorandum (AP)

“If indeed this is a Russian invasion of Crimea and if we do conclude the [Budapest] Memorandum is legally binding then it’s very difficult to avoid the conclusion that we’re going to go to war with Russia,” Brenton told BBC radio.

Ukraine has accused Russia of a “military invasion,” though details are still coming in.

The unsettling news comes after President Barack Obama warned Russia about military action in Ukraine on Friday.

But a closer look at the Budapest Memorandum shows the specifics might be more complex than some are assuming. Article one of the agreement states:

The United States of America, the Russian Federation, and the United Kingdom of Great Britain and Northern Ireland, reaffirm their commitment to Ukraine … to respect the Independence and Sovereignty and the existing borders of Ukraine.

Reaffirming a “commitment” to Ukraine’s borders and being legally required to go to war are two very different ideas.

There will surely be much debate over whether the Budapest Memorandum is a legally binding agreement requiring action. The U.S. going to war with Russia is likely a last resort for the Obama administration.

According to rferl.org, the Budapest Memorandum is a diplomatic memorandum, not an official treaty.

Barry Kellman, a professor of law and director of the International Weapons Control Center at DePaul University’s College of Law, told the website that the answers to questions about whether the agreement is binding are “complex.”

“That’s actually a much more complex question than it may sound. It is binding in international law, but that doesn’t mean it has any means of enforcement,” he said.

“The ‘Budapest Memorandum’ follows the Helsinki Final Act and essentially reiterates its provisions. There are confidence building measures and then a host of other broader obligations – primarily negative obligations. Don’t interfere,” the professor added.

Armed men described as Russian troops took control of key airports in Crimea on Friday and Russian transport planes flew into the strategic region, Ukrainian officials said, an ominous sign of the Kremlin’s iron hand in Ukraine. President Barack Obama bluntly warned Moscow “there will be costs” if it intervenes militarily.

The sudden arrival of men in military uniforms patrolling key strategic facilities prompted Ukraine to accuse Russia of a “military invasion and occupation” – a claim that brought an alarming new dimension to the crisis.

Unidentified armed men patrol outside of Simferopol airport, on February 28, 2014. Ukraine accused today Russia of staging an 'armed invasion' of Crimea and appealed to the West to guarantee its territorial integrity after pro-Moscow gunmen took control of the peninsula's main airport. (Source: AFP PHOTO / VIKTOR DRACHEV VIKTOR DRACHEV/AFP/Getty Images)

Unidentified armed men patrol outside of Simferopol airport, on February 28, 2014. Ukraine accused today Russia of staging an ‘armed invasion’ of Crimea and appealed to the West to guarantee its territorial integrity after pro-Moscow gunmen took control of the peninsula’s main airport. (Source: AFP PHOTO / VIKTOR DRACHEV VIKTOR DRACHEV/AFP/Getty Images)

Obama urged Russia to respect the independence and territory of Ukraine and not try to take advantage of its neighbor, which is undergoing political upheaval.

“Any violation of Ukraine’s sovereignty and territorial integrity would be deeply destabilizing,” Obama said.

“Just days after the world came to Russia for the Olympic Games, that would invite the condemnation of nations around the world,” he continued. “The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine.”

He did not say what those costs might be.

The Associated Press contributed to this report.