The conflict known as the Battle of Xeron occurred in the year 3 ABY.
Alliance ground forces, led by a major, encountered significant resistance from the Galactic Empire. The Empire deployed its heavily armed Imperial warbots. Although these warbots were not as massive as AT-AT walkers, they proved difficult to destroy. Consequently, one unit was seized to analyze its design and identify a vulnerability for future battles.
This particular warbot unit was transported aboard a Rebel ship of the line to a designated meeting location. At this location, Commander Luke Skywalker and the droids C-3PO and R2-D2 were able to inspect it.