An accident happens - it's not immediately visually clear how - and Banning is able to save the president's life, but not, shall we say, with optimum results all around. So when we flip to 18 months later (indicated primarily by young Connor ditching the glasses and the wig he was wearing prior), Banning is no longer part of the president's personal protectors - not that he ever should have been, as a best friend who likes punching the guy in the face, but y'know, whatever. We also learn that he told the Speaker of the House to go fuck himself, which is particularly bad due to the fact that said speaker is Morgan Freeman.
And then North Koreans take over the White House. Yes, North Koreans. They are all master-ninjas in this alternate reality, led by Rick Yune's master-terrorist Kang. Some of them are also suicide bombers, because the screenwriters don't give a shit about which of America's enemies are which. And their planes have secret pop-out weapons like MASK vehicles, which catch the U.S. air force totally by surprise. Really, they're more like VENOM than Kim Jong-Un's actual army.
After a massive firefight, and a tasteless bit in which the enemy plane crashes into the Washington Monument, only to have it collapse moments later like the WTC in miniature, Banning enters the White House through one of the holes blown into its side, and with the president sealed into the secret underground bunker with Kang, Banning is now the only inside man. In a blatant attempt to one-up John McClane, his black friend on the outside in this instance is the acting president, who is also...you guessed it...Morgan Freeman (the audience in my screening cheered maybe the loudest when he got sworn in).
Further cementing the fact that this is an alternate reality, we get MSNBC pundit Lawrence O'Donnell playing the news anchor on a network called KGDV, and at least one of the North Koreans has long hair and a beard. The CG flag the baddies toss of the roof flutters fakely on its way down, but who cares. Are you here to see Gerard Butler smash a man's head in with a bust of Lincoln and say things like, "Why don't you and I play a game of fuck off. You first!" or aren't you?
You don't have to answer that question in a public forum.