Earth's Terran Expeditionary Force Operations

What happens when two parallel worlds intersect at one point? What happens when two civilizations that have completely independent histories, cultures, and technologies but are extremely similar in many aspects intertwine and conflict? Once the war begins, it will be an all-out war in which all the fruits of civilization are staked, an ultimate war in which almost all lives in the two worlds are staked. This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.