WASHINGTON -- When NATO and Russia signed a historic cooperation deal in 1997, officials hailed the accord as a "victory for reason," a "definitive" end to the Cold War, and the dawn of collaboration in "a new Europe of unlimited possibility."

Nearly two decades later, that agreement, known as the NATO-Russia Founding Act, appears all but dead amid Moscow's intervention in Ukraine and the military alliance's push to boost its defenses on its eastern flank.

"Certainly de facto the agreement is not applying very well," Steven Pifer, a former U.S. ambassador to Ukraine, told RFE/RL.

The accord has been mired in mistrust and mutual accusations ever since Russia's annexation of Ukraine's Crimea territory in March 2014 and the war between Kyiv's forces and Russian-backed separatists in eastern Ukraine that erupted the following month.

NATO has accused Russia of essentially tossing the accord onto the scrap heap with its Ukraine intervention and responded by boosting its presence in the Baltics and Eastern Europe, Moscow's Soviet-era domain, where fears of potential further Russian expansion run deep.

Moscow calls those concerns groundless and says NATO allies are violating the tenets of the partnership act with troop deployments in the alliance's eastern member states.

Russia this week accused the United States of seeking to undermine a key provision of the pact after The New York Times reported that Washington is considering stationing tanks and heavy weapons in the Baltics and Eastern Europe.

The June 13 report said the proposal foresees "a company's worth of equipment -- enough for about 150 soldiers" -- being stationed in NATO's Baltic member states Lithuania, Latvia, and Estonia, while enough hardware "for a company or possibly a battalion -- about 750 soldiers -- would be located in Poland, Romania, Bulgaria, and possibly Hungary."

"By all appearances, the United States, in unison with its allies, is seriously determined to definitively undermine the key provision of the 1997 Russia-NATO Founding Act under which NATO undertook a commitment not to deploy substantial combat forces in the territories of countries in question on a permanent basis," Russia's Foreign Ministry said on June 15.

'Substantial' Debate

NATO has said it remains committed to the partnership accord with Russia, though some alliance members argue that the pact's provision prohibiting "additional permanent stationing of substantial combat forces" has been made obsolete by Russia's actions in Ukraine.

It says the rotation of troops through its eastern members constitutes a "persistent" rather than "permanent" presence in the region, and that the levels of these deployments are below the threshold of "substantial" included in the NATO-Russia Founding Act.

Russian officials, however, complain that the alliance has never provided them with a definition of what constitutes "substantial" combat forces under the pact.

Kevin Ryan, a former defense attache to the U.S. Embassy in Moscow, said recent back-channel talks he has led between retired Russian and American defense and intelligence officials suggest that for the Kremlin, "substantial" is tantamount to "any."

"At the last couple meetings, but especially at this last meeting in March, the Russians were clear to say that any increase in NATO forces and activity on their border is a threat, and Russia will not tolerate it," Ryan told RFE/RL.

Pifer, who served as senior director for Russia, Ukraine, and Eurasia on President Bill Clinton's National Security Council when the NATO-Russia Founding Act was signed, said NATO internally agreed at the time the pact was concluded on what constitutes "substantial combat forces."

Currently a senior fellow at the Brookings Institution, Pifer said that the "pre-positioned" equipment levels described in the New York Times report "are still well below that threshold," though he declined to provide details on the force levels that NATO originally considered to be "substantial."

"Since NATO never articulated that level to the Russians, the Russians will have a tendency to see anything that NATO does as substantial," he said. "But there was an internal alliance understanding that, again, there doesn't have to be any change to the Founding Act to put quite a bit more stuff into Central Europe and the Baltics."

White House spokesman Josh Earnest told reporters on June 15 that the proposal "is still working its way through the policy process and is in the early stages of the policy process."

"We want to make sure that NATO allies are defending their territory on a 24/7 basis, and we'll continue to support them and exercise vigilance in that regard," he said.

NATO defense ministers in February agreed to set up six command posts in Bulgaria, Estonia, Latvia, Lithuania, Poland, and Romania and create a spearhead force of up to 5,000 troops capable of deploying within two days in the event of a crisis.

They also agreed to boost the alliance's rapid-reaction force from 13,000 troops to 30,000 as part of what NATO Secretary-General Jens Stoltenberg has called "the biggest reinforcement of our collective defense since the end of the Cold War."

'La-La Land'

Some Western leaders, analysts, and former officials argue that Russia's revision of European borders has rendered the NATO-Russia Founding Act a geopolitical relic.

The accord's second paragraph, after all, states that NATO and Russia "do not consider each other as adversaries." Lower down in the document, the alliance's pledge to forego "additional permanent stationing of substantial combat forces" is preceded by the qualifier "in the current and foreseeable security environment."

"All the language of the document, nearly all of it has been overcome by events," Steve Saideman, a defense and NATO expert at Carleton University in Canada, told RFE/RL.

Lobbying for a permanent NATO base in his country, Estonian President Toomas Hendrik Ilves said on Twitter in August that those who believe this security environment has not changed since 1997 are "living in la-la land."

U.S. President Barack Obama last year also said that "the circumstances have clearly changed" since the pact was signed, language that Stoltenberg echoed at a June 16 briefing with reporters in Brussels.

"We are increasing our presence in the eastern part of the alliance as a response to the new and changed security environment and as a response to the more assertive behavior of Russia in Ukraine," Stoltenberg said.

Asked whether the U.S. proposal reported by The New York Times is in line with the NATO-Russia Founding Act, Stoltenberg said that "everything we do is fully in line with our international commitments and obligations."

He added that NATO defense ministers would discuss the proposal with U.S. Secretary of Defense Ash Carter at next week's summit.

With reporting by RFE/RL's Rikard Jozwiak in Brussels