Providing weapons while in an active conflict is generally considered taking a side in the conflict, especially if you give them for free.

This has a big impact because history has shown that basically you cannot defeat a people until they accept defeat. One of the ways to cripple an enemy and hopefully get them to give up the fight is to destroy their military production. When arms are supplied by another country, that puts their arms production out of range unless you add the supplier to the conflict and attack them, too.