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.
Tensions boil over between Russia and Ukraine in Black SeaNov. 26, 201800:49
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.
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.
"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.
"If Russia gets away with what it's doing and considers the costs acceptable, what does it try next?"
In 2016, Russian Foreign Minister Sergey Lavrov offered another explanation, claiming that the agreement only stipulated its signatories "won’t use nuclear weapons against Ukraine" — which is not true.
However, some have expressed concern that Poroshenko could be using the Azov crisis to weaken democracy as he trails badly in the polls ahead of elections slated for March.
Ukrainian President: 'Russia will pay a huge price if they attack us'Nov. 27, 201801:15
As well as invoking the Budapest Memorandum, he has also declared martial law, neither of which he did even at the height of the country’s crisis in 2014.
The problem is that the Budapest Memorandum is a political agreement rather than a legally binding treaty. It does not say countries have to take any particular action if it is violated, other than enter into talks.
The U.S. says it remains committed to the agreement, and has provided more than $2.8 billion since 2014 to help Ukraine "defend its territory and implement key reforms."
Some 200 U.S. troops are stationed at a base in western Ukraine, albeit hundreds of miles from Crimea or fighting in the east. And earlier this year the Trump administration sold Ukraine Javelin anti-tank guided missiles worth an estimated $47 million.
But Poroshenko told NBC News this week that what he really wants — "what we were promised" — is Washington's help to join NATO.
This would in theory give his country the alliance's Article 5 mutual-defense protection: an assurance that the U.S. and others would come to its aid in an attack.
U.S. sends Moscow a message by training with UkraineOct. 13, 201801:46
In September, Ukraine hosted NATO military exercises involving some 2,270 troops from 14 countries including the U.S.
However, analysts say there is little chance of Ukraine gaining NATO membership while the status of Crimea remains unresolved and conflict simmers in the east.
The Ukrainian president was seemingly snubbed again this week when he called on NATO to send its ships into the Azov and called on Germany to halt construction of the Nord Stream 2 pipeline, which is being built under the Baltic Sea and bypasses Ukraine.
Both NATO and Germany appear to have declined these overtures.
Just because they haven't gone to war over the issue, that doesn't mean Western countries have ignored the Budapest Memorandum, according to Jack Watling, a research fellow at the Royal United Services Institute.
"Policymakers haven't forgotten," he said, "but given that they've ruled out the most direct way of fulfilling that obligation, they are trying to use other tools in their arsenal: diplomatic, economic."
These tools include the raft of sanctions slapped on Russia by the U.S. and European Union since the Crimea annexation.
"There's no way of making this a limited war like the Falklands," Watling added, referencing Britain's war with Argentina in 1982. "It would be World War III and the risk of nuclear escalation is unacceptable. We're not prepared to take that risk over Crimea."
But Steven Pifer, a former U.S. ambassador to Ukraine and retired State Department official who was one of the memorandum's negotiators, said the U.S. and Europe should be doing more for Ukraine — both in terms of assistance and sanctions on Russia.
“It seems to me as much a matter of U.S. and European self-interest as a matter of the Budapest Memorandum: if Russia gets away with what it's doing and considers the costs acceptable, what does it try next?”