Complex sentence: The US Marine Corps Warfighting Laboratory started a Concept Demonstrator project in 1997 for a "mortar in a box", that is a weapon which could be remotely emplaced and fired unattended on a future battlefield.
Split into simpler sentences: The US Marine Corps Warfighting Laboratory started a Concept Demonstrator project in 1997. They wanted a weapon which would be a "mortar in a box," that is a weapon which could be remotely emplaced and fired unattended on a future battlefield.