Yugoslavia was invaded on April 6, 1941, and formal resistance barely lasted ten days. However, the German-led occupation proved particularly harsh, which contributed to a large-scale partisan campaign to fight against the Axis occupiers. Resistance was organized mainly by the communists under the leadership of Tito. What started as a guerilla war in the first three years evolved into more conventional warfare from the second half of 1944 onwards. By the war’s end, Tito’s Partisans numbered 800,000 and were organized into four field armies. The Partisans were a constant threat to occupying forces, and fighting was vicious as Axis troops attempted to neutralize this threat in a series of offensives.