As nations on both sides wanted to conquer more territories and build up colonies, those that were attacked were usually a 3rd world country and were already allied to the Allies or vice versa. This also helped spark the war for ww1, even tho the main reason was the assissination of Archduke of Austria-Hungary.
For ww2, many Axis nations didnt like the turn out of the Treaty of Versailles. Germany, becuase they had to pay a huge sum of money to the Allies, and literally became bankrupt.
Japan, for they did not get all the lands they wanted, and their European and American allies didnt want to hear them out.
Italy, because they also didnt get anything that they were hoping for.
During WW2, many nations called upon their colonies to help them fight the war. Some examples would be India and Britain, Egypt and Britain, etc etc.