Disclaimer: I do not own BTVS or Independence Day Resurgence. No profit is being made from this, it is purely for enjoyment, plus the plot bunny wouldn't leave me alone..
She wasn't always known as Elizabeth Lanford. Before she got roped into the work of politics, she had been the Slayer Buffy Summers. Then Sunnydale fell, the Hellmouth having been closed. There were no more vamps and no more demons, and the last of the alien menace had been neutralized in the Congo, so the world had no more need for the Slayer. What was left for her and the Scoobies now except a normal life?
Thankfully there were those such as David Levinson and former president Thomas Whitmore who saw the benefit in keeping them on retainer, and so the Scoobies were introduced to Thomas Lanford, and Giles's new Watchers' Council was rolled into Earth Space Defence; Giles himself didn't see much use in a supernatural deterrent to an alien resurgence, but it gave the Slayers something to do and a focus for their energies as the last leftover alien holdouts from the War of '96 were found and neutralized throughout the planet. Their agility and reflexes kept them out of the grasp of alien tentacles and their mystical preparation was proof against their telepathy. Their track record of eliminating alien nests was scarily good, so good, in fact, that when the ESD established one of their new bases on Saturn's moon of Rhea as part of Earth's new early warning system, nearly all the Slayers were posted there; supposedly the Slayers were frightening enough that most normal, "non-enhanced" humans didn't want them around.
No, even a normal life after demons and aliens was forever beyond the Slayers' reach.
Somehow Buffy had learned to redirect the lust for battle inherent in the Slayer into politics. She married Thomas Lanford and learned much from him, eventually going on to gain an endorsement by then-President William Gray, and becoming the running mate on Lucas Jacobs's bid for the Presidency. She did her job so well when she was known as Vice President Lanford, that she quickly forgot that she ever was Buffy Summers; an overwhelming majority voted Elizabeth "Buffy" Lanford into the Oval Office eight years later, just in time for the War of 2016.
Perhaps it was the long-dormant Slayer within her that gave her such a hawkish personality, or perhaps she was just possessed of a survivor's mentality. Regardless, when the first ship showed up over Luna base in the onset of the War of 2016, she chose to ignore Levinson's warning and authorized a first strike. When the Harvester aliens returned to finish what they started in 1996, President Lanford left Washington with what remained of her Cabinet and relocated to NORAD to prosecute the war effort from Cheyenne Mountain's more secure command facility. She had no delusions, though, as to how long NORAD would remain secure, and having the presence of mind to nominate General Adams as the designated survivor, took out her long-unused Slayer Scythe, waiting as she coordinated fleeing ESD units to form a counter offensive.
And when the Aliens broke through into the NORAD complex, President Elizabeth Lanford, formerly the Vampire Slayer Buffy Summers, hefted her Scythe in one hand and a blaster rifle in the other, and uttering these words made her last stand.
"There shall be no peace….."