They were able to unite because the Persian invasion of Greek mainland was a common threat for all, so they united to stop it. This was impossible before because Persia was not considered to be such a major threat.
It marked the end of absolute monarchies!
During World War I, Japan demonstrated its growing power by: using its large navy to capture German colonies in the Pacific and Indian oceans. It gave Japan an excuse to retaliate against the Chinese and gain control of Manchuria.
