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World War I

Mainly as a result of the increasing firepower (such as machine guns) available to defensive forces, First World War trench warfare briefly revived a form of siege warfare. Although siege warfare had moved out from an urban setting because city walls had become ineffective against modern weapons, trench warfare was nonetheless able to use many of the techniques of siege warfare in its prosecution (sapping, mining, barrage and, of course, attrition), but on a much larger scale and on a greatly extended front.

More traditional sieges of fortifications took place in addition to trench sieges. The Siege of Tsingtao was one of the first major sieges of the war, but the inability for significant resupply of the German garrison made it a relatively one-sided battle. The Germans and the crew of an Austro-Hungarian protected cruiser put up a hopeless defense and, after holding out for more than a week, surrendered to the Japanese, forcing the German East Asia Squadron to steam towards South America for a new coal source.

The other major siege outside Europe during the First World War was in Mesopotamia, at the Siege of Kut. After a failed attempt to move on Baghdad, stopped by the Ottomans at the bloody Battle of Ctesiphon, the British and their large contingent of Indian sepoy soldiers were forced to retreat to Kut, where the Ottomans under German General Baron Colmar von der Goltz laid siege. The British attempts to resupply the force via the Tigris river failed, and rationing was complicated by the refusal of many Indian troops to eat cattle products. By the time the garrison fell on 29 April 1916, starvation was rampant. Conditions did not improve greatly under Turkish imprisonment. Along with the battles of Tanga, Sandfontein, Gallipoli, and Namacurra, it would be one of Britain's numerous embarrassing colonial defeats of the war.

The largest sieges of the war, however, took place in Europe. The initial German advance into Belgium produced four major sieges: the Battle of Liège, the Siege of Namur, the Siege of Maubeuge, and the Siege of Antwerp. All four would prove crushing German victories, at Liège and Namur against the Belgians, at Maubeuge against the French and at Antwerp against a combined Anglo-Belgian force. The weapon that made these victories possible were the German Big Berthas and the Skoda 305 mm Model 1911 siege mortars, one of the best siege mortars of the war, on loan from Austria-Hungary. These huge guns were the decisive weapon of siege warfare in the 20th century, taking part at Przemyśl, the Belgian sieges, on the Italian Front and Serbian Front, and even being reused in World War II.

At the Siege of Przemyśl, the Austro-Hungarian garrison showed an excellent knowledge of siege warfare, not only waiting for relief, but sending sorties into Russian lines and employing an active defense that resulted in the capture of the Russian General Lavr Kornilov. Despite its excellent performance, the garrison's food supply had been requisitioned for earlier offensives, a relief expedition was stalled by the weather, ethnic rivalries flared up between the defending soldiers, and a breakout attempt failed. When the commander of the garrison Hermann Kusmanek finally surrendered, his troops were eating their horses and the first attempt of large-scale air supply had failed. It was one of the few great victories obtained by either side during the war; 110,000 Austro-Hungarian prisoners were marched back to Russia. Use of aircraft for siege running, bringing supplies to areas under siege, would nevertheless prove useful in many sieges to come.

The largest siege of the war, and arguably the roughest, most gruesome battle in history, was the Battle of Verdun. Whether the battle can be considered true siege warfare is debatable. Under the theories of Erich von Falkenhayn, it is more distinguishable as purely attrition with a coincidental presence of fortifications on the battlefield. When considering the plans of Crown Prince Wilhelm, purely concerned with taking the citadel and not with French casualty figures, it can be considered a true siege. The main fortifications were Fort Douaumont, Fort Vaux, and the fortified city of Verdun itself. The Germans, through the use of huge artillery bombardments, flamethrowers, and infiltration tactics, were able to capture both Vaux and Douaumont, but were never able to take the city, and eventually lost most of their gains. It was a battle that, despite the French ability to fend off the Germans, neither side won. The German losses were not worth the potential capture of the city, and the French casualties were not worth holding the symbol of her defense.

The development of the armored tank and improved infantry tactics at the end of World War I swung the pendulum back in favor of maneuver, and with the advent of Blitzkrieg in 1939, the end of traditional siege warfare was at hand. The Maginot Line would be the prime example of the failure of immobile, post–World War I fortifications. Although sieges would continue, it would be in a totally different style and on a reduced scale.