The sad truth about this extremely powerful ad, is that it is, in point of fact, actually true. Note, the ad doesn't accuse Trump of 'treason', that's a technical thing, but they are accusing him of betraying America, which he has.
According to my current and former military friends, the worst thing he did to them was to pull them out of Syria, leaving our staunch allies in the fight against ISIS to the "tender mercies" of that Turkish butcher, Erdogan. That decision to leave the Kurds hanging was one of the most shameful things our military has ever done. And Trump did it at the behest of both Erdogan and Putin. To the rank and file soldiers, that was pretty much the end of any respect they ever had for Trump.