Siege
Even when in the end the New Model Army—a regular professional army—developed the original decision-compelling spirit permeated the whole organisation, as was seen when pitched against regular professional continental troops the Battle of the Dunes during the Interregnum.

Experienced commanders on both sides in the English Civil War recommended the abandonment of garrisoned fortifications for two primary reasons. The first, as for example proposed by the Royalist Sir Richard Willis to King Charles, was that by abandoning the garrisoning of all but the most strategic locations in one's own territory, far more troops would be available for the field armies, and it was the field armies which would decide the conflict. The other argument was that by slighting potential strong points in one's own territory, an enemy expeditionary force, or local enemy rising, would find it more difficult to consolidate territorial gains against an inevitable counterattack. Sir John Meldrum put forward just such an argument to the Parliamentary Committee of Both Kingdoms, to justify his slighting of Gainsborough in Lincolnshire.
Sixty years later, during the War of the Spanish Succession, the Duke of Marlborough preferred to engage the enemy in pitched battles, rather than engage in siege warfare, although he was very proficient in both types of warfare.
On 15 April 1746, the day before the Battle of Culloden, at Dunrobin Castle, a party of William Sutherland's militia conducted the last siege fought on the mainland of Great Britain against Jacobite members of Clan MacLeod.

Strategic concepts: In the French Revolutionary and Napoleonic Wars, new techniques stressed the division of armies into all-arms corps that would march separately and only come together on the battlefield. The less-concentrated army could now live off the country and move more rapidly over a larger number of roads.
Fortresses commanding lines of communication could be bypassed and would no longer stop an invasion. Since armies could not live off the land indefinitely, Napoleon Bonaparte always sought a quick end to any conflict by pitched battle. This military revolution was described and codified by Clausewitz.

Industrial advances: Advances in artillery made previously impregnable defenses useless. For example, the walls of Vienna that had held off the Turks in the mid-17th century were no obstacle to Napoleon in the early 19th.
Where sieges occurred (such as the Siege of Delhi and the Siege of Cawnpore during the Indian Rebellion of 1857), the attackers were usually able to defeat the defenses within a matter of days or weeks, rather than weeks or months as previously. The great Swedish white-elephant fortress of Karlsborg was built in the tradition of Vauban and intended as a reserve capital for Sweden, but it was obsolete before it was completed in 1869.
Railways, when they were introduced, made possible the movement and supply of larger armies than those that fought in the Napoleonic Wars. It also reintroduced siege warfare, as armies seeking to use railway lines in enemy territory were forced to capture fortresses which blocked these lines.
During the Franco-Prussian War, the battlefield front-lines moved rapidly through France. However, the Prussian and other German armies were delayed for months at the Siege of Metz and the Siege of Paris, due to the greatly increased firepower of the defending infantry, and the principle of detached or semi-detached forts with heavy-caliber artillery. This resulted in the later construction of fortress works across Europe, such as the massive fortifications at Verdun. It also led to the introduction of tactics which sought to induce surrender by bombarding the civilian population within a fortress, rather than the defending works themselves.
The Siege of Sevastopol during the Crimean War and the siege of Petersburg (1864–1865) during the American Civil War showed that modern citadels, when improved by improvised defences, could still resist an enemy for many months. The Siege of Plevna during the Russo-Turkish War (1877–1878) proved that hastily constructed field defenses could resist attacks prepared without proper resources, and were a portent of the trench warfare of World War I.
Advances in firearms technology without the necessary advances in battlefield communications gradually led to the defense again gaining the ascendancy. An example of siege during this time, prolonged during 337 days due to the isolation of the surrounded troops, was the Siege of Baler, in which a reduced group of Spanish soldiers was besieged in a small church by the Philippine rebels in the course of the Philippine Revolution and the Spanish–American War, until months after the Treaty of Paris, the end of the conflict.
Furthermore, the development of steamships availed greater speed to blockade runners, ships with the purpose of bringing cargo, e.g. food, to cities under blockade, as with Charleston, South Carolina during the American Civil War.

Modern warfare: 

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.