Time Warrior Series by Matthew Costello

When grad student Jim Tiber hears about a time machine at Columbia University, his first plan is to go back in time and to meet the Beatles and study the way they rose from mediocrity. However, just as he's about to discover their secret in 1962, he is thrown back in time to an alternate timeline - Rommel's powerful North Africa campaign.

When the Iron Men alter the course of history, upsetting history's delicate equilibrium, Jim Tiber is pulled through a dangerous rip in time, and he must set the past right and stay alive. Original.