There is a movie called The Tomorrow War, which has the premise that in the year 2050 (or thereabouts) humanity is almost wiped out by an alien invasion. So they go to the only place they can to recruit more soldiers… the past!
But why not go to the future to get soldiers? They’ll probably have even more awesome technology to bring back with them, too, and the possibilities for genetic engineered super-soldiers are almost limitless.
It might be objected that the problem with going to the future to find the soldiers to save humanity is that until you save humanity, there are no people in the future to bring back. But there are people in the future because you’ve saved humanity, so you can go get them. But you can’t do that until you’ve already saved humanity! a friend of mine cried when I brought this up (or at least typed; he was too far away to hear how loudly). Ah, but this isn’t a problem in time travel, because there is a future for you to go to in order to bring back soldiers to save humanity because by the time they would have died off you’ve already brought soldiers from the future back to save them. Problem solved.
“OK, once this loop got going it can keep going, but what about the first time?” an eagle-eyed defender of the movie might ask. I’ve been assured by atheists that this is simply an invalid objection, though. Things can keep going forever without having to start. There just never was a time when the soldiers from the future didn’t go even further into their future to recruit soldiers to bring back into the past, and it keeps working because it already worked, without ever having to have gotten started. No matter how far into the past of the time-loop you go, it’s explained by the previous loop. Obviously, this is a highly satisfying explanation for a movie because atheists are quite satisfied with it for the universe.
Not to mention, if the people from 2050 can come to 2021 and convince everyone of the severity of the situation such that a world-wide draft gets instituted in 2021, why on earth is the answer to go forward to 2050 and fight in small groups alongside a tiny remnant of humanity against a mostly dominant alien force? Why not send everyone to 2049 alongside the still numerous humanity to try to overwhelm the invaders? Or why go forward in time at all? It would be far more effective to instead stay in 2021 and focus on building up humanity’s numbers and weapons stockpiles and such-like using the technology brought from the future to speed up development. It would make far more sense for the remnant of humanity from 2050 to come back to 2021 to help us prepare then fight alongside us than for us to go and fight alongside them. Or, rather, to warn us in 2021 then go back to 2045 to fight alongside us then.
All time travel stories intrinsically have plot holes in them, but I find it interesting—suggestive, even—that they so often make the worst decisions they can, given their premises. It’s almost like the sort of people who would tell time travel stories don’t really care about plot holes.