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.

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, Politico says.

Both Belarus and Armenia are members of the Collective Security Treaty Organization (CSTO), a post-Soviet military alliance led by Moscow and formed in 2002. Theoretically, members are obliged to defend each other if attacked. Azerbaijan quit a precursor to the bloc in 1999.

On Wednesday, Armenian Prime Minister Nikol Pashinyan announced his government would begin the process of withdrawing from the bloc, claiming its members were “not fulfilling their contractual obligations, but are planning a war against us with Azerbaijan.”

Now, a cache of more than a dozen letters, diplomatic notes, bills of sale and export passports 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.

Neither the Azerbaijani nor Belarusian governments responded to requests for comment.

However, according to experts, Belarus — one of Moscow’s closest allies — was unlikely to be acting without the tacit support of the Kremlin itself. “This truly shows that with friends like Vladimir Putin, nobody needs enemies,” said Ivana Stradner, a research fellow at the Foundation for Defense of Democracies.

“It’s ridiculous to think these transfers could have taken place without Moscow’s knowledge, and that Russia couldn’t have stopped them if it wanted to,” she added. “There is no such thing as loyalty when it comes to Moscow — it’s all about preserving their own security even if it’s at the expense of their own allies.”