A crossfire is a military term for the siting of weapons (often automatic weapons such as assault rifle

Assault rifle

An assault rifle is a selective fire rifle that uses an intermediate cartridge and a detachable magazine. Assault rifles are the standard infantry weapons in most modern armies...

s or sub-machine guns) so that their arcs of fire overlap. This tactic came to prominence in World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

.

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 mine

Land mine

A land mine is usually a weight-triggered explosive device which is intended to damage a target—either human or inanimate—by means of a blast and/or fragment impact....

s, sniper

Sniper

A sniper is a marksman who shoots targets from concealed positions or distances exceeding the capabilities of regular personnel. Snipers typically have specialized training and distinct high-precision rifles....

s, barbed wire

Barbed wire

Barbed wire, also known as barb wire , is a type of fencing wire constructed with sharp edges or points arranged at intervals along the strand. It is used to construct inexpensive fences and is used atop walls surrounding secured property...

, and air cover, crossfire became a difficult tactic to counter in the early 20th century.

Trench warfare

The tactic of using overlapping arcs of fire came to prominence during World War I where it was a feature of trench warfare

Trench warfare

Trench warfare is a form of occupied fighting lines, consisting largely of trenches, in which troops are largely immune to the enemy's small arms fire and are substantially sheltered from artillery...

. Machine gun

Machine gun

A machine gun is a fully automatic mounted or portable firearm, usually designed to fire rounds in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute....

s were placed in groups, called machine-gun nests, and they protected the front of the trenches. Many lives were lost in futile attempts to charge across the no man's land

No man's land

No man's land is a term for land that is unoccupied or is under dispute between parties that leave it unoccupied due to fear or uncertainty. The term was originally used to define a contested territory or a dumping ground for refuse between fiefdoms...

 where these crossfires were set up.

"Caught in the crossfire"

To be "caught in the crossfire" is an expression that often refers to unintended casualties (bystander

Bystander

A bystander a person who, although present at some event, does not take part in it; an observer or spectator.*Bystander effect, a social psychological phenomenon wherein individuals do not offer help in an emergency situation when other people are present...

s, etc.) who were killed or wounded by being exposed to the gunfire of a battle or gun fight, such as in a position to be hit by bullets of either side. The phrase has come to mean any injury, damage or harm (physical or otherwise) caused to a third party due to the action of belligerents.