"The outbreak of World War II was mainly due to the policy of appeasement." To what degree is this statement true?
The true cause of World War II was the actions of the aggressive powers, mainly Germany and Japan. However, we can argue that it was appeasement that allowed their actions to escalate to the point where war began. This is much truer of the European war than of the Pacific war.
The true blame for the war falls on Germany and Japan. It was their decision to actually do things like invading Poland or attacking Pearl Harbor. Regardless of what caused them to think these actions would work, they chose to take those actions. We would not say that I am morally culpable if I leave my car unlocked and you steal it. I was dumb, but you were the one who did the morally bad thing. The same is true of the Axis powers.
That said, appeasement helped to give the Axis powers the impression that they could do whatever they wanted. Allowing Hitler to remilitarize the Rhineland or to take the Sudetenland surely emboldened him to eventually invade Poland. Allowing Japan to invade China surely helped to make them think they could get away with other invasions (though it would have been much harder for the US to stop the invasion of China than for France or England to prevent the remilitarization of the Rhineland).
So, the actions of the Axis powers were truly responsible for starting the war. However, appeasement did help to convince the Axis that they could act aggressively without starting a war.