If you are in another country trying to understand what is happening in California because of Trump and ONLY because of Trump

If you study when the last time that a president of the U.S. put National Guard troops under federal mandate it was 1965 in Alabama by LBJ (President Lyndon Baines Johnson).

begin partial quote from Google AI:

The last time a President put the National Guard under federal control against the objections of a governor was in 1965, when President Lyndon B. Johnson federalized the Alabama National Guard to protect civil rights protesters during the Selma to Montgomery march. end quote.

So, normally the National Guard is under the direct Authority of the Governor of a state. So, to most Americans what Trump is doing is violating States Rights to local Sovereignty. 

Though we are a country of 50 States it is more like we are 50 Countries joined together by a Federal Government in times of War.

Since there is no war or real need to do what Trump has done it is authoritarianism sort of like Hitler we are watching and most Americans are horrified at Trump's Abuse of power and want him to stop this before a Civil War breaks out which it could starting in California from what Trump has already done.

Telling people he might arrest the Governor of California will Threaten all Governors in the U.S. that Trump is considering this.

Threatening STates Rights is no small thing by the Way.

Like I said the last time this was done by a president was 1965 in order to protect protesters from death and injury.

There is NO real justification for what Trump is doing now legally at all.

The last time a President invoked the Insurrection Act to suppress rioting and restore order following a request from a governor was in 1992 during the Los Angeles riots after the Rodney King verdict. 

The Posse Comitatus Act generally restricts the use of federal military forces for domestic law enforcement, but the Insurrection Act provides a legal exception in certain circumstances.