I'm looking for a child / YA book that featured time travel. It would have been published around or before 1975. I remember that it had some minor travel to the past, then travel to the future, to a military base in Asia (China, likely), in the winter and wearing suits that rendered them invisible, which failed when entering the building they were infiltrating.
I believe this is the book "Time Gate" by John Jakes (better known for his historical fiction such as the Kent Family Chronicles) from 1972.
If this is correct, you might remember that the group first went back to Ancient Rome tracking a "rogue" villain who believes it is possible to improve the present by changing the past. He uses the example of killing Hitler before WWII. However, the rest of the team argues that as horrible as Hitler was, changing the past could have unforeseen consequences. E.g. there might not have been the development of nuclear power which in this world has solved the energy crisis forever.
However, the The villain almost gets killed by a Roman legion when he causes problems. Ironically, he tries to stop the Legion by pointing a hand laser weapon at the soldiers. But, because the legionaires don't recognize it as a weapon, they simply take him captive. The team frees the villain and returns to the present (1987)
The villain then somehow escapes/breaks free and travels back in time to assassinate the President Archibald due to political differences. The team, fearing war, went to the future to try and understand what happened. They find a post-apocalyptic city--one of the very last--which while very advanced is dying out. The city historians believe it was the assassination of President Archibald which led to the nuclear war which had occurred.
They then travel back in time from the futuristic city to save President "Archibeld"--misspelling intentional as the future historians had made a mistake on the name. They succeed in saving the President. However. when they return, they find the war still occurred.
They then travel back in time to the Asian military base to stop the war from starting leaving one of the team members in the future at the hospital having been wounded in saving the President. This time they take the future City's leader's teenage daughter with them (who happens to be a computer and physics genius).
It is at this point they use the futuristic suits to infiltrate the base. I also remember a scene in which they spill rice wine all over the computer. It thought at the time this was very funny :-)
In the end,
The team succeeds in stopping the launch. When they return they find the war never happened and the world has changed beyond recognition. The daughter still exists. However, all her family has disappeared having never been born in that timeline.
I've found the book I had in mind. The title is "Time Gate" and has a chapter titled "The Red Door." It was published in 1972. My memory of the book matches what you described in your question.
I can't find a preview or a copy online, but the Google Books page for the book has a list of key words, including President Archibald. So, this is the book I remember, but I don't know for sure if it is the book you are looking for.