The Invisible City - Brian K. Lowe

Trapped behind enemy lines in World War I France, Lieutenant Charles Clee stumbles upon an illegal time-travel experiment and finds himself catapulted 850,000 years into the future.

Marooned in a nightmare world where prehistoric behemoths tread side-by-side with genetically-altered monsters from the laboratories of tomorrow, hunted by the eon-spanning Time Police and Earth's alien overlords, Clee races across the globe searching for the fabled last time machine-his only chance to go home.

If he returns, he can save the men of his command and turn the tide of the war-but if he remains, he can become the symbol of Earth's resistance to centuries of slavery.

In the end he must choose: Honor his oaths and rescue the men he swore to protect in the past, or honor his humanity and accept the leadership of mankind in the struggle to save the future?

No matter his decision, a world is doomed.