The old adage politics stops at the water’s edge means that

The old adage, “politics stops at the water’s edge” means that that a nation should work together to attain its common purposes in foreign strategy and approach.

If you have any other questions, please do not hesitate to ask me.

The Mexican-American War opened up the American Southwest and California to settlers. Prior to the mid-18th century war, Mexican territory encompassed what is now Arizona, New Mexico, and a bulk of California. However, following the Mexican surrender, the US laid claim to most of that land thereby opening the regions up to American settlers moving west.