president and governor's permission

In the case of foreign invasion the President could act without informing the governor. In fact, the President would be expected to act immediately or sooner.

King Neptune's right, V. Let's take a silly scenario.

Message from US President to Governor of Texas:
Understand you have been invaded by hostile troops. Intend sending fighter aircraft and paratroopers. Please confirm in order.

Message from Governor of Texas to US President:
No, Presidente. Would not be appreciated. Pleese to not send troops. Gracias.

It's likely (but I don't know for sure) one of the president's emergency powers. But since you bring it up, I'm not sure what the case is in the event of natural disasters. That's probably a recent precedent you could look up, since the US hasn't been invaded since Poncho Villa.

vigeo said:

OK, but what if the invader's whereabouts and intentions were not known by anyone but the president? Contact with the governor must be made, preferably before the assault, so emergency management can be arranged.

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If possible the President would inform the governor of the states involved, and the military commanders would try to coordinate with whatever services they would need, but the military has its own medics, etc., so they wouldn't need much.

The interesting thing here is "...what if the invader's whereabouts and intentions were not known by anyone but the president?"

What kind of invader would only be known to the President? If you are putting aliens or supernatural characters in, then things would not change. If this is your story, then you can have your president do whatever you want him to do. He can bee like W and think that the Constitution is just a piece of paper. The nice thing about writing fiction is that one can have one's character do anything at all.

King Neptune said:

If possible the President would inform the governor of the states involved, and the military commanders would try to coordinate with whatever services they would need, but the military has its own medics, etc., so they wouldn't need much.

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The President would end up informing the governor(s) ASAP. He doesn't need their permission (far as I know), but he does have to inform them about what's going on, so that they can inform their in-state generals and get the military that's already there ready to go. Also, so that whatever units he sent to help are ready to be received.

Trust me, by the time the invaders got there, the state and federal government will have talked the situation to death. That's the plus of telephones and internet.

The interesting thing here is "...what if the invader's whereabouts and intentions were not known by anyone but the president?"

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Yeah, this.

The US President can federalize the state's National Guard (usually under the command of the state's Governor) in times of emergency. The protocol is typically as simple as a phone call.

http://www.howstuffworks.com/national-guard.htm

http://en.wikipedia.org/wiki/National_Guard_of_the_United_States