I read this book (novel-sized) sometime in the last 1970s. I think it was called “Time Trap”. The title is too generic to get usable results from a websearch. I thought it was written by Ben Bova, but it’s not in a list of his works.
Some of the key plot points:
[li]In the mid 1970’s, America had been at war with German for 35 years. Germany was winning, having conquered the Eurasian continent 30 years before.[/li][li]President John F. Kennedy’s science advisors had detected some sort of time disruption, leading to the 1940s.[/li][li]He authorizes them to build a device that will take people back to investigate.[/li][li]The scientists that go back discover that although backward time travel will go back to your own past, that forward travel will put them in some random possible future (unless a receiving gateway is in the future)[/li][li]They discover their 1970s was not the original; that their reality was “created” when someone from another 1975 went back and gave Hitler technology to win the war, and keep winning. That someone, though, had the forethough to build a receiving doorway so that they could return.[/li][/ul]
Any ideas what book this is?