But we have not encroached on Mexican territory for quite a long time.

'Quite a long time' ?

Where I live was Mexico until 1850. Prior to statehood, guys like William Dana of Boston married California Mexicans to gain Mexican citizenship so they could get California land grants. Dana wanted Ojai or San Luis Obispo but got Nipomo instead. 'The Vaquero Song' mentions him.

Mexican communities were getting the shaft via eminent domain until... well, recently (see Chavez Ravine, aka Dodger Town).

1962 Dodger Stadium opened. Chavez Ravine families still resent being booted.
1967 Israel was getting attacked by its neighbors.

Not so long ago that the Arabs wanted to wipe Israel off the map and still do.

What do you think the Palestinians should do to reclaim the occupied territory?

If other nations sent arms and trained militias in Baja California to attack California Norte with waves of missiles from and regularly launch cross border incursions with suicide bombers hitting La Jolla and Newport Beach, you have to know Pendleton, Miramar and Naval Base San Diego would respond in a fashion similar to the IDF; blockades, barriers and armed responses.

It's probably too late to do anything now. Maybe if they had not aligned with nations that wanted to exterminate the Jews it would be different. As Albaby observed, Palestine failed to create "a security apparatus and a military" to keep the peace.

Were the Jews too harsh? What's too harsh when the only thing protecting you from Islamic zealots, keeping you alive, is a badass military?

Surely the Palestinians have a right to existence and territory too?
They've had a long time to create a state that would peacefully coexist.

Again, if your town was regularly plagued by suicide bombers.... The Pals made a horrific bed in which to lay. I truly feel sorry for those who are caught in the middle.