Q:

Looking for a time travel book where future generation travels and conquers the past

I am looking for the title of a book which I read in the 1960's.  It is about a time gate opening up form which an army from the future appears to conquer their past.  They are able to do so because of a book passed down from generation to generation describing how and when the enemy would attack.  
The Book cover had a time arch (like St. Lois arch) with rainbow colors.  
What is the name of this SF story? 

A:

The most likely candidate is INVASION FROM 2500 by Norman Edwards and published by Monarch Books in 1964.  And, yes, there is a glowing arch with a line of futuristic tanks emerging.  The novel has the following apt dedication: "To Terry Carr and Ted White, who made this book possible".  Terry Carr and Ted White collaborated under the pseudonym of "Norman Edwards", so they certainly made the book possible.  Your memory had added the fancy details of rainbow colors.  The arch on the cover is a golden color and surrounded by a bright white glow. 
The blurb on the back cover has this evocative quotation:
"Jack stopped the car and ran toward the light.  He was greeted by a fantastic sight: a huge, weirdly glowing arch, through which were pouring strange-looking tanks, airships and metallic-clad soldiers carrying sinister weapons." 
Hope this helps.