A main line of resistance (MLR) is the most important defensive position of an army facing an opposing force over an extended front. It does not consist of one

trench A trench is a type of excavation or in the ground that is generally deeper than it is wide (as opposed to a wider gully, or ditch), and narrow compared with its length (as opposed to a simple hole or pit). In geology, trenches result from ero ...

or line of pillboxes, but rather a system, of varying degrees of complexity, of fighting positions and

obstacle An obstacle (also called a barrier, impediment, or stumbling block) is an object, thing, action or situation that causes an obstruction. Different types of obstacles include physical, economic, biopsychosocial, cultural, political, technologic ...

s to slow enemy advances.

History of concept

The term first came into use during

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was List of wars and anthropogenic disasters by death toll, one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, ...

, after fighting became stalemated across northern France. The French and British on one side, and the Germans on the other, built elaborate fortified defensive positions. These were characterized by extensive use of

barbed wire A close-up view of a barbed wire Roll of modern agricultural barbed wire Barbed wire, also known as barb wire, is a type of steel fencing wire constructed with sharp edges or points arranged at intervals along the strands. Its primary use is ...

, entrenchments and underground

bunker A bunker is a defensive military fortification designed to protect people and valued materials from falling bombs, artillery, or other attacks. Bunkers are almost always underground, in contrast to blockhouses which are mostly above ground. T ...

s to protect their troops from enemy fire, and defeat enemy attacks. The depth of such positions could range from several hundred to several thousand meters, and in a few cases much farther. If the position was held in great depth, a screening line of strongpoints and fortified outposts - designed to slow and disorganize an enemy attack - might be constructed forward of the MLR, and a reserve line built behind it. The most famous and elaborate MLR of World War I was the Siegfried Line (part of the longer German

Hindenburg Line The Hindenburg Line (German: , Siegfried Position) was a German defensive position built during the winter of 1916–1917 on the Western Front during the First World War. The line ran from Arras to Laffaux, near Soissons on the Aisne. In 1916 ...

), across parts of northern France. During

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...

, in which combat was relatively fluid, the term ''main line of resistance'' was used less often, and the positions the term described were usually less deep and complex than in World War I. However, there were exceptions, including the French

Maginot Line The Maginot Line (french: Ligne Maginot, ), named after the Minister of the Armed Forces (France), French Minister of War André Maginot, is a line of concrete fortifications, obstacles and weapon installations built by French Third Republic, F ...

, the German

Atlantic Wall The Atlantic Wall (german: link=no, Atlantikwall) was an extensive system of coastal defences and fortifications built by Nazi Germany between 1942 and 1944 along the coast of continental Europe and Scandinavia as a defence against an anticip ...

and

Westwall The Siegfried Line, known in German as the ''Westwall'', was a German defensive line built during the 1930s (started 1936) opposite the French Maginot Line. It stretched more than ; from Kleve on the border with the Netherlands, along the wes ...

(Siegfried Line to the Allies), as well as the Soviet defenses at the

Battle of Kursk The Battle of Kursk was a major World War II Eastern Front engagement between the forces of Nazi Germany and the Soviet Union near Kursk in the southwestern USSR during late summer 1943; it ultimately became the largest tank battle in history ...

. After the

Korean War {{Infobox military conflict , conflict = Korean War , partof = the Cold War and the Korean conflict , image = Korean War Montage 2.png , image_size = 300px , caption = Clockwise from top:{ ...

became static in 1951, MLR described the defensive positions of the U.N. Eighth Army, a series of trenches and bunkers extending east to west across the Korean peninsula. {{Fortifications Land warfare