Belarus delivered advanced weapons to Armenia’s arch enemy even though both countries were supposedly allies in a Russian-led international defense pact, according to leaked documents seen by POLITICO.

The cache of files sheds new light on Armenia’s decision this week to announce it will be leaving the military alliance, a dramatic turn that will weaken Russian President Vladimir Putin’s authority with former Soviet nations.

Armenia is now on the verge of making a historic tilt toward the West, increasingly looking toward Europe and NATO for protection, after decades in which the former Soviet republic relied on Moscow.

Armenia has been locked in a bitter conflict with Azerbaijan in the South Caucasus region, at the strategic junction between Asia and Europe, frequently boiling over into full-scale war.

The decision by Belarus — a staunch ally of Russia — to supply advanced military hardware to Azerbaijan between 2018 and 2022, giving it the upper hand in a spate of wars with its long-standing rival, will have been regarded as a bitter betrayal by Armenia.

Now, a cache of more than a dozen letters, diplomatic notes, bills of sale and export passports seen by POLITICO shows that Belarus actively aided Azerbaijan’s armed forces between 2018 and 2022, as tensions peaked with Armenia. The services offered included modernizing older artillery equipment and providing new gear used for electronic warfare and drone systems.

The documents include letters from the Belarusian state arms export agency to its own military-industrial firms relating to orders of state-of-the-art artillery targeting equipment for Azerbaijan as well as correspondence between the two states agreeing the purchase of Groza-S counter-drone mobile warfare stations for Azerbaijan’s armed forces.