Biden played a key role in NATO's expansion east in the '90s, which Putin is now threatening to invade Ukraine over

• Putin blames NATO's eastward expansion for the tensions with Ukraine.
• Biden played a key role in expanding NATO in Eastern Europe during the late 1990s.

"This, in fact, is the beginning of another 50 years of peace," Democratic Sen. Joe Biden of Delaware declared in 1998 as the Senate voted in favor of expanding NATO to include Poland, Hungary, and the Czech Republic.

Biden, the top Democrat on the Senate Foreign Relations Committee in those days, said adding the former Cold War enemies to the Western military alliance amounted to "righting an historical injustice forced upon the Poles, Czechs, and Hungarians by Joseph Stalin."

The Washington Post in its report on the Senate vote described Biden as a "key player in the ratification effort." Indeed, then-Senator Biden was among the loudest voices in championing NATO's expansion in Eastern Europe in the late 1990s. He would continue to support NATO expansion into the 2000s as one of the most influential senators in Washington and later as vice president.

Fast-forward to 2022 and Russian President Vladimir Putin is threatening to entangle Europe in a new war over precisely the issue Biden once claimed would foster decades of peace. When the Soviet Union collapsed, some Warsaw Pact countries it had dominated cast their lot with NATO — moves that Russian leaders saw as weakening their influence and ability to defend against European invasions like those mounted by Napoleon and Hitler.

With roughly 100,000 troops on Ukraine's border, Putin is demanding that the former Soviet republic be blocked from ever joining NATO. Though the Kremlin claims it has no plans to invade Ukraine, the Biden administration has warned that a Russian military incursion into the former Soviet republic could be imminent.

A crisis 30 years in the making

Putin has a history of aggression toward Ukraine. Russia invaded Ukraine in 2014 — annexing Crimea in the process and its port on the Black Sea — and has since supported rebels in a war against Ukrainian forces in the eastern Donbass region.

The Russian leader's decision to assault Ukraine that year followed massive protests that led to the ousting of the country's pro-Russian president, Viktor Yanukovych, who lives in exile now in Russia.

The Russian president has said that Ukraine is "not even a country," and called Ukrainians and Russians "one people." He's made his ambitions in the region quite clear: he wants the US out of Europe, and Ukraine firmly under Russian control.

Putin is "deadly serious" about taking action on Ukraine, Fiona Hill, who served as the top Russia advisor on the National Security Council under the Trump administration, told Insider in November. "One way or another, he wants Ukraine neutralized," Hill added, underscoring that Putin views Russia's next-door neighbor as unfinished business.

Meanwhile, Putin has blamed NATO's eastward expansion for the contentious dynamic between Moscow and the West.

It's a geopolitical hostage crisis — and neither side is backing down. The US and its allies have made clear that NATO's open door policy is non-negotiable, while Russia refuses calls to pull troops from Ukraine's border.

Putin, a former KGB operative, has ruled over Russia for 20 years. Over that time, he's modernized Russia's military and vied to reestablish Moscow's authority in countries that were once part of the Soviet Union. The Russian leader's bellicose posture toward Ukraine — and anger toward NATO — is fundamentally linked to these ambitions.

"The current crisis between Russia and Ukraine is a reckoning that has been 30 years in the making. It is about much more than Ukraine and its possible NATO membership. It is about the future of the European order crafted after the Soviet Union's collapse," Angela Stent, a senior fellow at the Brookings Institution and a former US National Intelligence Officer for Russia and Eurasia, recently wrote in Foreign Affairs.

"During the 1990s, the US and its allies designed a Euro-Atlantic security architecture in which Russia had no clear commitment or stake, and since Russian President Vladimir Putin came to power, Russia has been challenging that system," Stent added.

Putin rewrites history

Putin on Tuesday said NATO "swindled" Russia, claiming Moscow was "given promises not to move NATO infrastructure to the East, not a single inch." The Russian president has repeatedly accused NATO of breaking such a pledge, but experts say Putin is distorting history in order to justify aggression toward Ukraine.

"[Putin] claims that NATO took advantage of Russian weakness after the collapse of the Soviet Union to enlarge to its east, in violation of promises allegedly made to Moscow by Western leaders. But no such promises were made," Steven Pifer, the US ambassador to Ukraine from 1998 to 2000, wrote in 2014 for Brookings Institution.

Though it's true the US in 1990 floated the idea of halting NATO expansion to the East during discussions with the Soviet Union on German reunification, there was never a formal agreement in this regard.

"I don't think Putin is all that worried about historical accuracy," Mary Sarotte, a historian who wrote a book on NATO expansion, "Not One Inch: America, Russia, and the Making of Post-Cold War Stalemate," recently told the New Yorker.

"At one extreme, there's a position you sometimes hear from the American side, that none of this ever came up, it's a total myth, the Russians are psychotic," Sarotte said. "On the other end, you have the very adamant Russian position: 'We were totally betrayed, there's no doubt about it.' Unsurprisingly, when you get into the evidence, the truth looks to be somewhere in between."

Biden: 'I helped lead the effort to enlarge NATO'

In the wake of World War II, NATO was founded by the US and its allies to protect Western Europe from the Soviet Union. Since the collapse of the USSR and the end of the Cold War, many former Soviet republics have joined NATO, including Hungary, Poland, the Czech Republic and Romania. Biden, as both a senator and vice president, repeatedly supported this process.

As chairman of the Senate Foreign Relations Committee in March 2008, for example, Biden expressed pride "that, here in the Senate, I helped lead the effort to enlarge NATO," adding that it remained his "conviction that we should extend an offer of NATO membership to any country that applies and meets the criteria."

At the time, Biden was expressing support for expanding NATO's presence in the Balkans by adding Albania, Croatia, and Macedonia as members. He also championed setting Ukraine and Georgia on a path toward NATO membership. Russian troops, under Putin's orders, would go on to invade Georgia just months later in August 2008.

As vice president in 2009, Biden said that the US supported Ukraine's move to join NATO — regardless of objections from Russia.

With Putin incensed over NATO expansion and the US eager to prevent a new conflict in Europe, Biden as president has taken a more cautious tone on the topic than he did as a senator or vice president. Though Biden has emphasized that the US firmly backs NATO's open door policy, he's tiptoed around the question of accepting Ukraine into the alliance.

"School's out on that question," Biden told reporters in June when asked about Ukraine joining NATO. "It remains to be seen."

Given Ukraine is not a NATO member, Biden has ruled out sending in troops if Russia invades. But Biden is planning to send thousands of troops to Poland and Romania, both NATO allies, in the coming days. Tensions between Moscow and the West haven't been this high since the Cold War, and it's an open question as to what happens next.