The main reason was joining the Axis Powers, which earned them territory.
The Arabs
The Byzantine Arab wars were some of the longest lasting in history. They mostly pitted Arab Muslims against Eastern Roman or Byzantine Empire between the 7th and 11th centuries AD. The wars started during the initial Muslim conquests
The answer is: The sea
Back then, the techology that exist to travel by sea was not as developed as it is today. If an empire decided to travel through the sea in order to do some conquests, that empire will risk several things, such as:
- Loss in directions
- The ships could be swallowed by harsh wheather in the middle of the sea
- It need a lot of resources to maintain the armies during the travel