Why was Germany and Japan's relationship considered a marriage of convenience?

Both Germany and Japan were both military dictatorships. Their foreign policies didn't conflict. They were both opposed to Great Britain and France and their influence in the world as well as the Soviet Union and Communism. Germany was given a free hand in Europe and Japan was given a free hand in Asia. Although their spheres of conquest didn't overlap they cooperated when they could. While Great Britain was being bombed by Germany, the Japanese captured Hong Kong, Singapore and Burma in Asia. These were British colonies in Asia. The Japanese sunk British ships like the "Prince of Wales" when they were able to. The Japanese took over Indo-China, that was a colony of the French. After the Japanese attack on Pearl Harbor and America was in the war against that nation, Germany declared war on the United States. So the "axis" between Japan and Germany was a marriage of convenience. What would have happened if they had won the war is anybody's guess. I hope that this has been helpful.