Alright here's the scenario:
The President, The Vice President, the Executive Cabinet, the White House, the Pentagon, the Joint Chiefs of Staff, the Senate, and The House of Representatives have all been killed or destroyed in two precision nuclear attacks, and a coordinated effort involving two massive sniper attacks, effectively eliminated any survivors, along with their families.

The snipers were killed before any information could be extracted from them, and all you know is that they are of Israeli origin; however the Israelis are denying any involvement.

One of the nukes, due to the nature of the attack, and the blast radius, was able to be tracked back to an "entity" known to have sold weapons of mass destruction to both Iraq and Iran.

Ironically, the North Koreans claim responsibility, however all intelligence collected from spying on them over the years clearly points to their inability to coordinate the attack.  To top it off, though, a Soviet nuclear class submarine went missing in the East Pacific and no allied agency can locate its whereabouts.  Sources claim the submarine, however, was in fact sold to the North Koreans in exchange for a large shipment of cargo approximately a year ago (the contents of which where never discovered).  One thing for sure is the submarine is armed with two more nuclear warheads ... the size of which is currently under debate considering the deaths of such high profile people.

Adding to the chaos, Mexican Federales, apparently augmented with more weapons and ammunition than previoiusly thought to have in their arsenal, crossed the US-Mexico Borders in Texas, New Mexico, Arizona and California and are requiring the return of Texas, New Mexico, Arizona, and California to their country or they will execute every man, woman, and child, from the border to as far north as they can go, as reciprocity for the "stealing" of their land.

Your Involvement:
Previously you were the Senior Aid to the National Security Advisor.  Although among the surviving individuals of the attacks there are people senior to you, you are the most respected among them, and widely seen as the "man/woman" for the job to step up as the new President.  In this "hypothetical" you accepted the nomination and were sworn in as President.

Priority One - Retalitory Measures : Where do you start?  The Generals need orders for the military, the CIA has a private military in waiting for "secret" orders in conjuction with OGA (Other Government Agencies), and the British SAS and the French Foreign Legion have offered their assets/manpower to help you (again hypothetical).  You have two nuclear warheads ready for immediate launch, which can reach any land in the world.  You have 3 others, however they won't be available for 1 hour .... and they happen to be located in what appears to be the most probable target of any further attacks.  Who does what?

Priority Two - Preservation of a Governing Body : Before you leave the office you need to pick a chain of command to continue after you, in the event that another attack ends your life.  You only have enough time to pick 10 people (all will say yes ... again hypothetical).  Who's with you?

(Air Force One, and all of your other assets for your position were not affected by the strikes ... you still have them .... and you have two aircraft carrier fleets already deployed ... one in the Mediterranean and one in the Red Sea area, and they are ready for immediate deployment).

***The UN Security Council is largely inneffective due to the vast amount of finger pointing going on between the differing allegiances among the member countries, and it appears there may be another division similar to the 'Axis and Allies' of WWII, so you will get no help from them.***
..................
Try not to think for too long on this ... give yourself about 5 minutes to think it over and than get yourself into gear.  Armeggedon may very well be upon you and it's better to put up a fight than die in a library!  Ready, Set, Go!