The Ypres Salient is the area around Ypres in Belgium which was the scene of some of the biggest battles in World War I.
In military terms, a salient is a battlefield feature that projects into enemy territory. Therefore, the salient is surrounded by the enemy on three sides, making the troops occupying the salient vulnerable. The enemy's line facing a salient is referred to as a re-entrant (an angle pointing inwards). A deep salient is vulnerable to being "pinched out" across the base, forming a "pocket", in which the defenders of the salient become trapped, isolated and easier to overcome. This gives attackers an overwhelming advantage: defenders can be attacked from all sides with artillery and/or machinegun fire. Also, since the defenders are surrounded, they cannot easily be re-supplied (with food, ammunition and medical supplies etc.) or escape. The "pocket" progressively reduces in size as the defenders are worn down and the attackers advance. The decreasing size of the pocket allows even more concentrated gunfire to be aimed at the defenders. Eventually, the defenders are overwhelmed by this onslaught and the pocket collapses.
The Ypres salient was formed by British, French, Canadian and Belgian defensive efforts against German incursion during the 1914 "Race to the Sea", culminating in the Battle of the Yser and the First Battle of Ypres.
These battles saved the Ypres salient and the corner of Belgium around Veurne from occupation, but also led to the beginning of trench warfare in the salient as both sides "dug in" around the line. The area of the salient is mostly flat, with few rises or hills. Those that did exist became the focus for the 1915 Second Battle of Ypres, which saw the first use of gas and the almost total destruction and evacuation of Ypres, and the 1917 Third Battle of Ypres at Passchendaele.After the third battle, the Ypres salient was left relatively quiet until the Fourth Battle of Ypres, the Battle of the Lys, when the Spring Offensive threatened to overwhelm the entire area. This offensive was stopped at the point the Allies were closest to being forced to abandon the salient. By August 1918, the Fifth Battle of Ypres (part of the Hundred Days Offensive) pushed the German forces out of the salient entirely and they did not return.