There really is no Hallmark Card for ordering a nuclear attack on an ally country because of an alien invasion. That is exactly what [the fictionalized] President Richard Nixon did in the third issue of the comic book Doctor Who Prisoners of Time celebrating the 50th Anniversary of Doctor Who.
So, could the [fictional] President Nixon simply order a nuclear attack on Great Britain? First, let’s understand the basic facts of the story.
The story so far has focused on the first three Doctors.
There is a hooded villain with weathered skin wearing a Vortex Manipulator kidnapping the Doctor’s different traveling companions.
I suspect that the final story will have all the Doctors saving all of the traveling companions from whoever the villain is (It might be the Valleyard given the personal nature of the kidnappings, but the Master or another classic villain are possibilities).
The third issue of Prisoners of Time had members of UNIT taken over by Remoraxians, who were trying to flood the planet with rain storms, causing the oceans to rise. A CIA Agent helping the Third Doctor tells President Nixon of a failed rescue, thus Nixon orders the attack on Great Britain to save the Earth.
The US Constitution, Article I, gives the Congress the power to declare war. Article II of the Constitution names the President at the Commander-in-Chief of the military. In 1973, after Vietnam and the Tonkin Gulf Resolution, Congress passed the War Powers Act over President Nixon’s veto. The relevant part of the Act states:
Presidential Executive Power as Commander-in-Chief; Limitation The constitutional powers of the President as Commander-in-Chief to introduce United States Armed Forces into hostilities, or into situations where imminent involvement in hostilities is clearly indicated by the circumstances, are exercised only pursuant to (1) a declaration of war, (2) specific statutory authorization, or (3) a national emergency created by attack upon the United States, its territories or possessions, or its armed forces.
There is still debate whether Congress could limit Presidential power, which has not yet been litigated (the debate about using Drones in Libya without Congress authorization highlights the issue is still alive).
As to our story, there is no mention of President Nixon consulting Congress other than ordering the bombers (most likely B-52’s) to conduct a nuclear attack on Great Britain. However, the attack arguably was legal under section three of the quoted War Powers Act passage, because the Remoraxians had attacked the United States, and the rest of the planet, with a weather attack. This created national emergency, enabling the President to order an attack without Congressional authorization….and ironically help save the world.
Spoiler: They did turn the bombers around and did not nuke England. Good thing Nixon did not launch ICBM’s.