While it comes across like the film is taking whatever that brand of creative license is called, but the famous general and soldier slapper really did believe that he had seen millennia of conflict.
Admittedly, he also wrote poems with titles like "The Turds of the Scouts," but his belief in reincarnation was legit, and he spent both World Wars claiming to know his way around old European towns and battlefields thanks to his time as a 14th-century French knight. Unfortunately, Sir Patton was one of many Frenchmen to meet his end at the Battle of Crecy, a crushing victory for the English, but it would have been suspicious if he'd claimed nothing but a string of huge successes.
His propensity for past lives began when he was kicked by a horse as a young man and, through the haze of his difficult recovery, saw a vision of himself as a dying Viking raider. Exactly how Patton reconciled this with his devout Christian beliefs remains unclear, but he also claimed that he was at the Siege of Tyre, served under Caesar and Marc Antony, fought with the English at Agincourt because apparently when you reincarnate during a lengthy conflict, you can ask to switch to what looks like the winning side, served the House of Stuart during the English Civil War, and was a bigshot in Napoleon's Grande Armee. Weird that he never just popped up in some trivial border skirmish, but maybe that just wasn't worth commenting on.
Some of Patton's other poetry paints war as a bureaucratic slog, but he sure seemed to enjoy it on a personal level given that his "Through a Glass Darkly" concludes with the belief that he'll continue to fight "forever in the future." And while Patton was beloved by his troops, they may have had a somewhat different view if they knew his fearlessness stemmed from the belief that he'd soon be right back in business no matter what happened. Anyway, this may all sound ridiculous, but reincarnationresearch.com thinks he's currently living as James Mattis, and surely if anyone would know, it's those guys.