Infiltration tactics
Development during World War I: These tactics emerged gradually during World War I. Several nations modified their existing tactics in ways that supported ideas that were later called infiltration tactics, with the German developments having the most impact, both during the war and afterwards.

Germany: As far back as the 18th century, Prussian military doctrine stressed maneuver and force concentration to achieve a decisive battle (Vernichtungsgedanke).  The German military searched for ways to apply this in the face of trench warfare.  Captain Willy Rohr fought in the long Battle of Hartmannswillerkopf (1914–1915), starting with two Pionier (combat engineer) companies. Such engineers were often employed in assaulting fortifications, using non-standard weapons and tactics compared to the regular infantry.  Rohr's initial efforts to use these as special advanced strike teams, to break French trench lines for following troops to exploit, achieved only limited success, with heavy losses.  Rohr, working with his superiors, saw equipment improved, including the new Stahlhelme (steel helmets), ample supplies of hand grenades, flamethrowers, light mortars and light machine guns.  Rohr's analysis was that much more training was needed to incorporate the new weapons and to coordinate separate attacks as needed to achieve the overall operational goals.  His analysis got the attention of the Oberste Heeresleitung (OHL, German army high command).  In December 1915, Rohr was given the task of training the army in "modern close combat", and soon promoted to major.  During the next two years, special Stoßtruppen (stormtrooper) detachments were created in divisions throughout the army; select men were sent to Rohr for training, who became trainers when they returned to their units. These tactics were expanded and refined by many in the German military command, extending the Prussian military doctrine down to the smallest units – specially trained troops maneuvered and organised to strike selected positions, wherever opportunities were found.

German infiltration tactics are sometimes called Hutier tactics, after German General Oskar von Hutier, even though his role in developing the tactics was limited.  Hutier, along with his artillery commander Colonel Georg Bruchmüller, improved the use of artillery in ways that suited infiltration tactics.  Conventional mass-wave tactics were typically preceded by days of constant bombardment of all defender positions, attempting to gain advantage by attrition.  Hutier favoured brief but intense hurricane bombardments that allow the opponent little time to react and reinforce their line.  The bombardment targeted the opponents' rear areas to destroy or disrupt roads, artillery, and command centres.  This was done to suppress and confuse the defenders and reduce their capability to counterattack from their rearward defence lines.  For maximum effect, the exact points of attack remained concealed until the last possible moment, and the infantry attacked immediately following the short bombardment.: 158–160 
The German stormtrooper methods involve men rushing forward in small but mutually supporting groups, using whatever cover is available, and then laying down covering fire for the other groups as they moved.  The tactics aim to avoid attacking any strongpoints directly, by first breaching the weakest points of the defender's line, and using those to gain positional advantages on other points.  Additionally, they acknowledge the futility of managing a grand detailed plan of operations from afar, opting instead for junior officers on the spot to exercise initiative, expanding the earlier Prussian doctrine of Auftragstaktik (mission-based tactics).
Due to the extensive training needed, stormtroopers remained small elite forces.  Regular infantry with heavy weapons would follow up, using more standard tactics, reducing isolated and weakened opposing strongpoints with flank attacks, as the stormtroopers continued the advance beyond them.  Reserve troops following these had to consolidate gains against counterattacks.: 157 

The Germans employed and improved infiltration tactics with increasing success, at first defensively in counterattacks as part of Germany's defence in depth and then offensively, leading up to the Battle of Caporetto against the Italians in 1917 and finally the massive German spring offensive in 1918 against the British and French.: 489   Initial German successes were stunning; of these, Hutier's 18th Army gained more than 50 km (30 mi) in less than a week – the farthest advance in the Western Front since the Race to the Sea had ended the war of movement in 1914.  This advance would hereafter associate Hutier's name with infiltration tactics in Western Europe.  The German armies began to stall after outrunning their supply, artillery and reinforcements, which could not catch up over the shell-torn ground left ruined by Allied attacks in the Battle of the Somme in 1916; the offensives failed to achieve a war-winning breakthrough dividing the French and British armies.: 137   The exhausted German forces lost the initiative and were soon pushed back in the Allied Hundred Days Offensive, ending in the German surrender.
Though far more successful tactically than traditional attacks, infiltration tactics did not address supporting any resulting advances operationally, so they tended to bog-down over time and allow the defender time to regroup.  German artillery, critical during the initial assault, lagged far behind afterwards.  The elite stormtroopers took notable casualties on the initial attacks, which could not be readily replaced.  German forces lacked mobile forces such as cavalry to exploit and secure deep advances.  Most importantly, German logistical capabilities, designed for a static front, failed to sustain troops advancing far into devastated enemy territory.
The German military did not use the term infiltration tactics as a distinct new manner of warfare but more as a continuous improvement to their wide array of military tactics. When the "new" German tactics made headlines in Allied nations in 1918, the French published articles on the "Hutier tactics" as they saw it; this focused more on the operational surprise of the start of the attack and the effective hurricane bombardment, rather than the low-level tactics. In post-war years, although information on "Hutier tactics" was widely distributed in France, the US and Britain, most generals were skeptical about these new tactics, given the German defeat. In Germany, infiltration tactics were integrated into the Reichswehr and the Wehrmacht. Felix Steiner, former officer of the Reichswehr, introduced the principle of stormtroopers into the formation of the Waffen-SS, in order to shape it into a new type of army using this tactic. When combined with armoured fighting vehicles and aircraft to extend the tactics' operational capabilities, this contributed to what would be called Blitzkrieg in the Second World War.

France: New French tactics that included an initial step for infiltration were published by the Grand Quartier Général (GQG, French General Headquarters) on 16 April 1915, in But et conditions d’une action offensive d’ensemble (Goal and Conditions for a General Offensive Action), its widely circulated version being Note 5779. It states that the first waves of infantry should penetrate as far as possible and leave enemy strongpoints to be dealt with by follow-up nettoyeurs de tranchée (trench cleaner) waves. The note covers weapons and close-combat tactics for the trench cleaners, but the tactics and weapons of preceding waves are unchanged, and there is little mention of any additional support for the now-detached advanced waves. The note contains annexes covering different subjects, including artillery, infantry defense, and infantry attacks. For attacks, Note 5779 continued to promote the pre-war French doctrine of la percée (the breakthrough), where an offensive is driven by a grand, single plan with continuous waves of reserves targeting the operation's distant and static objectives. It does not cover methods of adapting to local success or setbacks, nor the small-unit initiative, coordination and additional training this would require. The tactics were employed with some success on the opening day of the Second Battle of Artois, 9 May 1915, by the French XXXIII Corps; they advanced 4.5 kilometres (2.8 mi) in the first hour and a half of the attack but were unable to reinforce and consolidate to hold onto all these gains against German counterattacks. The battle was costly and inconclusive, taking a heavy toll in French troops and matériel. Later French infantry tactics moved away from the costly la percée towards a more practical grignotage (literally nibbling, taking in small bits) doctrine, which employed a series of smaller and more methodical operations with limited objectives; each of these were still planned at headquarters, rather than from immediate local initiative. Note 5779 also describes an early form of rolling barrage in its artillery annex; this was employed with success and continued to be developed by the French as well as by most other nations throughout the war.
In August 1915, a young French infantry officer, Captain André Laffargue, put forward additional ideas in a pamphlet titled Étude sur l’attaque dans la période actuelle de la guerre (Study of the Attack in the Current Period of the War). Laffargue based his proposals in particular on his experiences in the initially successful but ultimately disappointing results of employing the tactics of Note 5779 at the Second Battle of Artois; he commanded a company of the 153rd Infantry Regiment, attacking immediately south of Neuville-Saint-Vaast on 9 May 1915. Laffargue was left wounded on the German front line but his regiment advanced another 1.5 kilometres (0.93 mi), only to be held up by two German machine guns.