Why did Alita Mars Alliance hit the Earth?

A plan to prevent the earth from uniting with the government. According to the setting in the movie Alita: Fighting Angel, the reason why the Mars Alliance attacked the Earth was mainly to stop a plan of the Earth Coalition government for Salem. During the 200 years from 2 1 century to 23/century, this plan started the human solar system migration plan, sent spaceships and humans to the major planets in the solar system, and gradually established the colonial planets of Venus Alliance, Jupiter Alliance and Mars Alliance.

In this context, the Mars Alliance chose to attack the Earth in order to stop this plan of the Earth Coalition government.