A bridgehead is a High Middle Ages
High Middle Ages
The High Middle Ages was the period of European history around the 11th, 12th, and 13th centuries . The High Middle Ages were preceded by the Early Middle Ages and followed by the Late Middle Ages, which by convention end around 1500....

 military term, which antedating the invention of cannons was in the original meaning expressly a referent term to the military fortification
Fortifications are military constructions and buildings designed for defence in warfare and military bases. Humans have constructed defensive works for many thousands of years, in a variety of increasingly complex designs...

 that protects the end of a bridge
A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...

. Like many older terms, the meaning of the word drifted with the passage of time, becoming used for something not exactly true to its initial usage.

With the introduction of cannons, the term was morphed to a generalized term for field fortifications lying some distance beyond the ends of the bridge that were emplaced to protect both the bridge and any troops crossing it to the far bank, so became in that era a term used when referring to both the fortifications and the small lodgement on the bank that was closest to the enemy. As the process of moving an army over bridges is slow and complicated, it is usually necessary to secure it from hostile interruption, and the works constituting the bridge-head must therefore be sufficiently far advanced to keep the enemy's artillery out of range of the bridges—hence as artillery grew in power, so did the size of the lodgements necessary. In addition, room is required for the troops to form up on the farther bank. Formerly, with short-range weapons, a bridge-head was often little more than a screen for the bridge itself, but modern conditions have rendered necessary far greater extensions of bridge defenses.

Then armies and military formations grew, so needed more lodgement area to organize a force large enough to stage a break out against a determined enemy, and again the technical meaning of the term expanded, again referring to a large fortified area about at bridge end. With the advent of modern warfare capabilities including long range tube artillery and high powered rifles with effective ranges measured in the thousands of yards (metres) the term of art again had to expand in area, but now morphed to be just an area defended and controlled by ample firepower, with or without constructed fortifications.

The term has been generalized in colloquial usage to refer to any kind of defended area that is extended into hostile territory – also called a foothold and sometimes the technically incorrect 'beachhead
Beachhead is a military term used to describe the line created when a unit reaches a beach, and begins to defend that area of beach, while other reinforcements help out, until a unit large enough to begin advancing has arrived. It is sometimes used interchangeably with Bridgehead and Lodgement...

' which is frequently mistaken colloquially as synonymous terminology. The technical term refers in particular to the specific area on the farside of a defended river bank or a segment of a lake or riverine coastline held by the enemy forces, such as the Bridge at Remagen
Ludendorff Bridge
The Ludendorff Bridge was a railway bridge across the River Rhine in Germany, connecting the villages of Remagen and Erpel between two ridge lines of hills flanking the river...

. The term is especially applied when such a territory is initially seized by an amphibious assault with the tactical intent of establishing a supply line across the geographic barrier feature to allow further operational manoeuvring. In that sense, it has much in common with the popular misconception of a beachhead.

Bridgeheads typically exist for only a few days, the invading forces either being thrown back or expanding the bridgehead to create a secure defensive lodgement
A lodgement is an enclave taken by and defended by force of arms against determined opposition made by increasing the size of a bridgehead, beachhead or airheadOxford English Dictionary lodgement, lodgment "3. The action of establishing oneself or making good a position on an enemy's ground, or...

 area, before breaking out into open country – as happened when the U.S. 9th Armored Division
U.S. 9th Armored Division
The 9th Armored Division was an armored division of the United States Army in World War II. In honor of their World War II service, the 9th was officially nicknamed the "Phantom Division."-History:...

 seized the Ludendorff Bridge
Ludendorff Bridge
The Ludendorff Bridge was a railway bridge across the River Rhine in Germany, connecting the villages of Remagen and Erpel between two ridge lines of hills flanking the river...

 at Remagen
Remagen is a town in Germany in Rhineland-Palatinate, in the district of Ahrweiler. It is about a one hour drive from Cologne , just south of Bonn, the former West German capital. It is situated on the River Rhine. There is a ferry across the Rhine from Remagen every 10–15 minutes in the summer...

 in 1945 during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

. In some cases, such as during the Gallipoli Campaign 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...

, a bridgehead may exist for months.
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