The most precious resource on Planet Earth is also one of its rarest. Element 119, tachium, has two unique properties: first, it defies all predictions by having a naturally-occurring stable nuclide, and second, it is the only known fuel for time machines.
Or it was. Only one tachium-232-bearing site was ever found, in a remote site in the Canadian Arctic, and only a total of 200 grams of pure tachium were ever extracted from it. The ore was fully mined out within five years and another source was never found.
Time machines use tachium very efficiently, each drive core requiring only microscopic amounts per year traveled, but they do use it up. And of course owning a working time machine is a strategic necessity for the world powers - do you want to be the sucker with no defense when Russia decides to go back to 1814 and change Francis Scott Key's lyrics to "The Fart-Mangled Banner"? This became a problem. The time trips necessary for scientific research, international saber-rattling, and media opportunities were running on a finite resource.
There were only 15 grams of tachium remaining in existence when the United States unilaterally invaded the past. They'd scanned the next thousand years of future already, and they knew there wasn't any tachium to be found there. And besides, the future had more advanced technology and the benefit of hindsight; it was well-equipped to defend itself from the past. So the US sent a small special-forces expedition back to the Canadian Arctic, the day the mine was discovered, with a mission to immediately dispose of the original prospectors, extract all 200 grams of tachium, and return to the present.
It worked. All the tachium arrived in their time successfully. They now had 200 grams (the other 15 having never existed) for future trips. Plus a working time machine, operators, and carefully compiled documentation to compensate for the fact that, as a side effect of the expedition, tachium had never been available for use until that moment. But that was fine, they could re-invent time machines then and resume/begin operations where they (apparently, according to the documentation) left off.
Five years later, a bit hurried along by all the stakeholders frantically submitting competing requisitions for time machine time, the United States again ran low on tachium. That was fine, though, it was now in friendly hands in the past so no war would be necessary. The expedition team could just pop back to when the mine closed out, pick up all 200 grams of tachium from their past selves, and return to the present with everything needed to re-re-invent the time machine.
Five years later...
I know this story sounds silly, because of course tachium does not exist and never has. It won't exist five years from now, either. And it won't exist five years from that. And it won't exist five years later.
Tachium will exist, someday, if it ever stops being stolen forwards in time. If the people of the future ever fail or forget or choose not to rob the past to fuel the present, tachium will begin existing five years before that moment. It will last for five years, and be entirely consumed, and we'll never get it back.
And for all of the rest of history, those will have been the only five years when time machines were real.