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Twenty-five years ago, Ukraine was the world's third-largest nuclear power, with more warheads than the United Kingdom, France and China combined.

The government in Kiev inherited this arsenal after the breakup of the Soviet Union, finding itself in possession of an estimated 5,000 nuclear weapons, more than 170 intercontinental ballistic missiles and several dozen nuclear bombers.

In 1994, Ukraine agreed to dismantle this stockpile in return for a promise from Russia that the country wouldn't be attacked.

But after Russian forces fired at and seized three Ukrainian naval vessels on Sunday, Kiev has pointed to this deal and suggested that the U.S. and Europe should do more to protect it against the vastly superior Russian military.

For months Ukraine has accused Russia of restricting access to its own ports in the nearby Sea of Azov, alleging the Kremlin wants to turn it into a Russian lake.

But the attack on the vessels and detention of their crews brought the relationship between the neighbors to a new low. Ukraine says the incident occurred in international waters.

Ukraine insists Moscow is again blockading the sea, something Moscow denies. Russia says it merely needs to inspect all ships passing through as a security measure to protect a $3.6 billion bridge it's built across the Kerch Strait from its mainland to Crimea, which it annexed in 2014.

It’s against this backdrop that Ukraine has invoked the Budapest Memorandum on Security Assurances, which was signed in 1994 by Ukraine, Russia, the U.S. and U.K.

"Ukraine gave up the third-largest nuclear arsenal" in the world, Ukrainian lawyer and human rights activist Stanislav Batryn told UkrLifeTV this week. "Today Russia violated the territorial integrity of Ukraine and is in fact starting a Third World War."

After declaring independence in 1991, Ukraine found itself with thousands of Soviet nuclear weapons that were still controlled by Russian systems.

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The U.S. was extremely worried about the potential emergence of another nuclear power with ICBMs designed to target the U.S. and its allies. So Washington brokered the agreement and paid half a billion dollars for Ukraine to pass these weapons to Russia to be dismantled.

In return, Russia, the U.S. and the U.K. agreed to "refrain from the threat or use of force" against Ukraine and to respect its "independence and sovereignty and the existing borders." Belarus and Kazakhstan also gave up their smaller stockpiles in exchange for the same promises.

When Russia annexed Crimea in 2014, it was widely accepted that this agreement had been violated. After this week's clash on the Black Sea, Ukraine used the memorandum as a rallying cry.

U.S. Defense Secretary William J. Perry, left, shakes hands with Ukraine Defense Minister Valery Shmarov, center, and Russian Defense Minister Pavel Grachev as they stand over the crater which formerly housed a missile silo at a military base near Pervomaysk, Ukraine, on Jan. 5, 1996. Yefrem Lukatsky / AP file

"We are appealing the entire pro-Ukrainian world coalition — we have to join our efforts," Ukrainian President Petro Poroshenko said Monday. "We are appealing to our partners in Budapest Memorandum that took on obligations to defend the sovereignty and territorial integrity."

Russian President Vladimir Putin's explanation in 2014 was that the memorandum had been agreed with a previous Ukrainian government, so was no longer valid. Most observers dismissed this as ridiculous; if agreements expired with the governments that signed them, then countless important treaties throughout history would be now void.