When the Trump administration declared it would walk away from the Open Skies Treaty in six months time, it left the door open for renegotiation.
There has been pressure mounting from all sides for the United States to salvage the arms control treaty.
The pact enables member nations to fly their aircraft over each other's territory, getting a clear picture of military activity.
For years the agreement has been used as a vehicle for transparency, but cost of the flights and alleged Russian foul play has pushed the US away.
Russia has joined UN officials and US opposition to the President in criticising the pullout.
Here is why it matters...
What is the treaty?
President Dwight Eisenhower first proposed that the United States and the former Soviet Union allow aerial reconnaissance flights over each other’s territory in July 1955.
Mutual reconnaissance flights were seen as a means of securing military transparency and arms control - avoiding escalation in the Cold War period.
Moscow rejected the idea at first, but President George HW Bush revived it in May 1989, with the treaty entering into force in January 2002.
Since then, more than 1,500 unarmed flights under the agreement have enabled nations to gather security data from military exercises and arms activity.
Why it is breaking down?
A statement from the US Department of Defence blamed Russia for America's withdrawal.
It believes Russia uses the treaty to justify aggression and spread propaganda and fears it "may use it for military targeting" against the US and its Allies.
The statement cites Russia's refusal and limitations of flights within its territory since 2017, "preventing the exact transparency" the pact is meant for.
In September 2019, Russia allegedly denied fellow Open Skies partners access above a major military execise.
China were among several nations to join Russia on Exercise Tsentr, which saw more than 120,000 personnel conduct drills on and around the Caspian Sea.
“Russia didn’t adhere to the treaty,” Mr Trump said.
"So until they adhere, we will pull out, but there’s a very good chance we’ll make a new agreement or do something to put that agreement back together."
Russian Deputy Foreign Minister Alexander Grushko told state news agency RIA the nation had not violated the treaty and has the right to continue talks on technical issues.
Moscow and Beijing are both in new nuclear arms control talks with the US, which could potentially replace an expiring weapons treaty with a three-way accord.
Strained relations could stall progress, while Mr Trump appears to be remaining positive the Open Skies Treaty could be rehashed.
Washington has also said imagery collected during the flights can be obtained quickly and at less cost from US or commercial satellites.
Why is it important?
Currently, the UK is one of 34 nations to have signed the pact - Kyrgyzstan is not yet ratified.
In an age of faltering arms control, the UN has warned a failure to replace Open Skies intelligence could lead to "a dangerous new arms race leading to possible miscalculations".
Flights are often carried out at short notice and, while the US could likely produce similar images using its satellites, but some nations cannot.
There are fears the same behaviour the US is accusing Russia of could be the very reason to salvage the pact rather than walk away.
Democrats in the US House of Representative and Senate have said withdrawal could undermine European nations reliant on the montoring of Russian military activity.
However, officials in Moscow have also criticised the move - Russian Deputy Foreign Minister Alexander Grushko calling the decision a "blow" to European security.
As things stand, the Americans will leave the Open Skies Treaty by the end of the year, by which time the US will have held another presidential election.
Cover image: US Open Skies aircraft in 2015 (Picture: US Department of Defense).