In my most recent review of "Tomorrow War," I went over those elements of the movie that I liked and why. While I was writing that review, I was thinking about this review from a "why the heck did they do that" perspective. You see, while the movie really is pretty damned good, there are holes within the plot that still left me scratching my head and asking, "why didn't you just do that?" If you haven't seen the film, there are major spoilers ahead as it will be impossible to tackle certain aspects of the plot without detailing them. You have been warned.
I going to preface this by stating the obvious- this is a movie and we have 20/20 vision regarding the events therein. Because we know how this plays out, it's easy for us to go back and say, "why didn't you just do X" when the characters and institutions within the movie didn't have that luxury. The script called for such and such to happen, and the pieces of those within the plot had to bend to the will of the screenwriter to make sure it happened. They don't get the option to alter the course of the stories events- their fate is sealed.
Actually- no, they're not. You see, we're dealing with a movie about messing with time. Perhaps before those being assaulted by the White Spikes figured out how to move backward in time they were a slave to the plot, but after they made their time bridge, that in ability to alter events changed dramatically. They weren't tethered to fate in the same way that we appear to be today. There are things that the people of the future knew that they could easily have passed backwards in time to those that they were requesting help from.
As an example, the mechanisms of time travel itself. This is a massive advantage that they have, and bringing that tool backwards in time with all the technology that is inherent within it would be a massive boon to mankind's ability to fight the White Spikes. Not just because suddenly you give mankind the ability to alter the future through altering the past, but because there are obvious leaps that were made in technological theories and abilities in 30 years that, if accomplished 30 years earlier, would give mankind a head start before the invasion ever occurs. Time travel itself as it is used in one such advantage with knock on capabilities that most people don't think of.
What do I mean? Read this post here for a more detailed breakdown of the problem with time travel here, but suffice to say in order to move between points in time you also have to move between points in space. The movie does a good job of sidestepping this problem with how time travel is envisioned, and therein lies the distinct advantage that their method of time travel gives them. Teleportation.
It may sound like a small thing, but within the context of time travel, you will have to cover a lot of space to go from point to point in time. By tethering two locations in time to one another you effectively make a bridge between them. If this bridge concept were applied to traveling between two points in space- say Earth and Mars- you could move between those two bodies at will. No longer would you need rockets to go into space, all you'd need was a sufficient power source. This offers a HUGE advantage within the confines of the situation mankind finds itself in during the movie. Transporting that ability backwards in time and giving people 30 years to establish a colony off Earth- a lifeboat if you will- to preserve the species.
This also allows for mankind to fortify space. Knowing the general location of the invasion, as well as knowing the aliens weaknesses and limitations, exploiting the ultimate in high ground is even more important. Placing orbital defenses over the area you know the attack is coming from thirty years in advance gives mankind the ultimate check to mate the enemy with, say, orbital delivered strategic nuclear weapons. The aforementioned colony off Earth is another huge advantage as there is no travel time between the two planets anymore, meaning all the required supplies to get the colony self sufficient could be sent at almost no cost to anyone- and things could be sent BACK to earth. That strategic nuclear strike to punch the White Spikes into oblivion could be delivered from Mars the moment the attack is located.
Additionally, with the ability to leap from point to point at will, establishing a base of operations in the area where the attack is to come from is a far simpler matter. This allows for exploitation of the region which, in turn, would probably lead to the discovery of the alien vessel where the White Stakes are being held in hibernation- allowing mankind to destroy them at their source without having to repel the invasion in the first place. All those are pretty big knock on effects of a simple "time machine," wouldn't you say? But all that doesn't answer the question of, "why send people forward in time to fight a war that would be far easier to prepare for in the past?"
The biggest plot hole in my opinion was this idea of sending soldiers forward in time to fight the war rather than information backwards in time to isolate where the invasion starts, and prepare to smash it when it arrived after thirty years of preparation. People and materials can both be transported across the time bridge, so why not send samples of the creatures as well as the computers necessary to do the hard work of analysis into the safety of the past? All the research taking place deep behind enemy lines, or on the oil platform could be done anywhere in space or time. The concerns of creating crazy time problems have already been proven false within the context of the movie- why not shelter the important stuff where the White Stakes can't hope to threaten it? There is no need to go forward to fight the war, and no need for the technology to stay in the future as well. Everything is split wide open- so embrace it.
Granted, all this only works if time functions in a way that would allow for us to alter the future without creating an infinite looping regression wherein we alter the past- change the future and then cause the past to revert because those who would have altered the past never go back to alter it in the first place. I'm operating on the assumption that either A) this isn't a thing, or B) every possibility is played out in time so it's allowed to happen. On the one hand, time is far more malleable than we think, or there are a plethora of alternate realities wherein every option is played out. So, rather than "change" the future for those in the future, you alter the current course of our own time creating a split. Whatever the case, that's a little beyond my pay grade at the present, so I'll just leave it to the reader to ponder.
Suffice to say, there are some options that mankind could have taken that were superior to "bring people from the past forward into the future to fight the hopeless war." Getting to the battlefield first, creating a lifeboat, and utilizing teleportation are all things that could have been used to great effect within the confines of the world created- and that's totally ignoring how those technologies could be applied to individual weapons, armor, and equipment. The White Spikes are a nasty foe to be sure- but could they handle a million powered armored soldiers? With teleportation? With a base of operations millions of miles away? I'm leaning toward, "no." Spend thirty years developing those tools to give mankind a distinct advantage in the fight, and what began as a route wherein mankind is exterminated turns into something far different- perhaps the key to FTL being unlocked by reclamation of the ship stuck in ice?
Whatever the route one takes in this thought experiment, it's easy to see that there are a plethora of options available to the human species within the film that offer far higher probabilities of success/survival were they to be implemented. This isn't to say that the writing was lazy, just that there are lots of things to consider in every situation. Looking toward alternate paths forward in a tough situation could lead to unintended and beneficial consequences that we'd only see in hindsight. This is one reason I love fiction- it gives us the ability to see potential futures and react accordingly. Perhaps our doom lies buried in the ice of northern Russia just waiting for the world to thaw and let it lose. Perhaps we are already the target of an invasion from space and it's only a matter of time before the assault begins.
Whatever the reality of the situation, we have options now to protect against those eventualities before they become realities. We don't have the hindsight to guide us- but we do have the hindsight of the past and fictional events to give us ideas to tackle whatever may come our way. Perhaps we should start using them.