Turn 1
Try to find a way to broadcast messages to the aliens.
5-1=4: You call up a buddy from NASA and ask him about broadcasting messages to outer space. He tells you that they have some equipment that might work, but they never got a chance to test it due to huge budget cuts. You ignore the accusatory tone in his voice and ask him to bring it over to Capitol Hill ASAP.
DAMMIT MAN! this is no time to talk to the enemy, we need to ready all troops and get ground to air missiles in every major city in America!
2+2=4: You dial up the Secretary of Defense and ask him about readying the troops and setting up missile defenses in major cities. He tells you that he'll talk to the President and Joint Chiefs of Staff about it, but no promises.
First priority: Get Ground to Air missiles into Washington and get the president into the bunker!
1-1=0: You run around screaming "GET MISSILES INTO THE PRESIDENT AND WASHINGTON INTO THE BUNKER!" People look at you funny.
Take all precautions. Have every single satellite, drone or any other government observation gizmo trained on any alien activity.
5-1=4: All the satellites and drones are watching for alien activity. Bill Clinton talks excitedly while looking through a pair of binoculars. He says he sees alien activity! You remind him that Colombian prostitutes are not the right kind of aliens to be watching and call the Secret Service members in the hotel across the street, telling them to get their asses back to work.
Prepare opening diplomatic message to the aliens. Set a peaceful, but professional tone.
1+1=2: "HEY ALIEN DUDES LET'S PARTY AND STUFF!"
Offer a broadcasted invite to the Aliens indicating an intent to set up cultural and diplomatic exchange programs.
1+2=3: "Greetings, aliens. Let us get to know each other by experiencing each other's culture. We will send you our finest Colombian prostitutes. Please send us some hot females of your kind."
Luckily, the broadcasting equipment isn't here from NASA yet for you to send that message.
A massive trailer pulls up in front of Capitol Hill. It opens up to reveal some very-expensive-looking broadcasting equipment from NASA.
4.5 hours remain...
So, I decided to be a bit loose with the definitions of combat and non-combat rolls. If it's anything involving fighting or getting ready to fight, it's a combat roll. Everything else is non-combat.