The primary reason for the Axis powers' success during this period of time was the fact that they were militarily superior to most other countries in Europe. By 1939, they had the second largest air force in the world with 8295 aircrafts. Only Russia, (10382 aircrafts), had a larger air force. The German's and the Russians were allies at this time and so Germany didn't view Russia as a threat. There was a similar scenario with troops. Hitler began a policy of forced conscription and began building a massive army, second in size to nobody. With this kind of military power at his disposal, Hitler would have found it difficult not to be successful. He could have conquered his enemies with sheer force.
Another reason Germany were so successful was because of their advanced military tactics. They employed a strategy called Blitzkrieg meaning "lightning war." The central ideas behind this tactic were co-ordination and speed. The tactic consisted of rapid thrusts by motorized divisions and tanks supported by air power. When the German's employed this tactic against their enemies, they were all defeated in a remarkably short time. Holland only survived 4 days before surrendering. Belgium went on a bit longer and survived about 20 days. And the almighty France' fought for just 43 days before surrendering. With superior tactics like this, it is little wonder why Germany was so successful in her conquests.
One of the main reasons for German victory over this period of time was the fact that her enemies were weak and ill-prepared for war. Holland, Belgium, Poland Denmark and Norway were all weak countries, not known for their military strength. Hitler found it easy to invade these countries as they had a fraction of the military power he had. They were all quickly defeated by a mixture of speed, co-ordination and sheer strength. They were defenceless to do anything about it as their armies were tiny compared to