Maybe the shortest invasion, but the USA invading an ally, and a NATO ally specifically, would have repercussions long after the invasion itself. It could result in the dissolution of NATO altogether which would greatly embolden Russia, and perhaps China and Iran, in their ambitions. I don't think Trump is particularly concerned about all that, which is the scary part.
In short, it might be the shortest invasion, but it might be the start of another long war involving most of the world. Which doesn't seem like a great way to "protect the free world".