A crossfire (also known as interlocking fire) is a military term for the siting of weapons (often automatic weapons such as assault rifles or sub-machine guns) so that their arcs of fire overlap. This tactic came to prominence in World War I.
Siting weapons this way is an example of the application of the defensive principle of mutual support. The advantage of siting weapons that mutually support one another is that it is difficult for an attacker to find a covered approach to any one defensive position.
Use of armour, air support, indirect fire support, and stealth are tactics that may be used to assault a defensive position. However, when combined with land mines, snipers, barbed wire, and air cover, crossfire became a difficult tactic to counter in the early 20th century.