@Radagast
A defunct British airline ending the Civil War would be a really weird time travel story.
BOAC Flight 777, shot down by Germans on June 1, 1943 in the Bay of Biscay, was never found. The plane skipped off the surface of the water, went back in time 80 years, crashing into the White House. Abraham Lincoln was entertaining his Secretary of War, William Stanton, and his Vice-President, Hannibal Hamlin, as well as Generals Hooker and Meade. They and their staffs were all killed in the resulting fire and building collapse.
When Lee began moving north on June 3 as he'd planned, towards Culpepper Court House, the Corps Commanders couldn't agree on what to do. The Union cavalry fell back in disarray instead of fighting at Brandy Station. Longstreet and Hill headed north along the mountains as planned, but when word reached them of the tumult in Washington, instead of heading west at Middleburg, Virginia, they headed east, with Ewell covering their northern flank while Meade's Army dithered near Frederick, and Hooker's Army headed north back to Washington, leaving Fredericksburg behind. On July 1, 1863, Lee's men found themselves overlooking Washington, and over the next two days of combat, managed to work their way through the defenses and the dispirited troops.
At that time in history, the US Presidential Succession was the Vice-President, and there was no clear plan for continuation of the government. The House and Senate had been busily meeting, fighting amongst themselves since the mysterious explosion that had taken out the civilian and military leadership, ignoring the warnings that Lee was closing on Washington until it was too late for them to evacuate. Some members tried to do so by sea, but once the Confederates seized some of the naval batteries, Confederate ironclads were able to blockade the Potomac. Members of Stuart's cavalry broke the rail line to Baltimore, trapping Congress.
With no way out, and to protect their own lives, even with overwhelming troop majorities, the US Congress surrendered to Robert E Lee on July 6th, 1863, ending the First American Civil War.