Question

the federal government also must protect each state from what ?

Answer 1

The United States shall guarantee to every State in this Union a Republican Form of Government,
(The Federal Government has to allow the individual States to govern themselves, it has to let them have their own government and laws)

and shall protect each of them against Invasion;
(The Federal Government has to protect the individual States from invasion from other countries--this is done by the Armed Forces)

and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence.
(This is usually done by the National Guard, and if one State were to attack another one (like Texas did to New Mexico during the Civil War) the Federal Government has to help if the individual State asks for it). This was done when Kennedy mobilized the National Guard to desegregate ole Miss.