“The Proteus Operation” is a time travel/alternate history novel by James P Hogan. The basic plot- History through World War I proceeds as normal. In the aftermath, though, it really becomes the war to end all wars. Peace and prosperity is established worldwide by 2020.
Spoilers for the overall plot of the book, though few highly specific details.
Then some assholes don’t like it, because they want more than their fair share, so they use a time machine to go back to 1920 so they can advise and support Adolf Hitler in his rise to power. Basically as soon as Hitler gets nukes, he cuts off the time portal and take over the world.
Cue the 1970s. North America, Australia, some of South America are free. The rest of the world is under Axis domination. The USA figures out what the hell happened, and begins a project to build their own time machine, knowing that they are about to lose when World War III starts, which is likely to be very soon. They build one. It’s not as good as the first one, but it will have to do. In 1975, President John F Kennedy sends a team to 1939. They manage to cut off the original time portal before Hitler has a decisive advantage, and history proceeds as we know it to have.
Interestingly, the survivors of the 1975 team stick around. One of them even sees his younger self.
Which implies that any of the 2020 time travelers who were in the past when the gate was destroyed… were left in place. They could have easily survived the war, and decide to bide their time until an opportunity presents itself to finish their original mission. They find their chance in a real estate developer from New York City…