Did Buddhists ever actively participate in a major violent conflict?
Suggested reading: Buddhist Warfare - Michael K. Jerryson and Mark Juergenmeyer. A view of the contents:


Making Merit through warfare according to the Arya-Bodhisaattva-upayavisaya-nirdesa-sutra
Sacralized Warfare: The Fifth Dalai Lama and the discourse on religious violence.
Soldier-zen in wartime Japan
Buddhists in China during the Korean War
Onward Buddhist Soldiers: Preaching to the Sri Lankan Army
Militarizing Buddhism: Violence in Southern Thailand