Peaceful penetration was an infantry tactic used toward the end of the First World War by Australian troops, a cross between trench raiding and patrolling. The aim was similar to trench raiding (namely, to gather prisoners, conduct reconnaissance, and to dominate no man's land), with the additional purpose to occupy the enemy's outpost line (and so capture ground).

The term came most directly ^{[clarification needed]} from the pre-war British press's description of the advancing penetration of German trade into the British Empire as "peaceful penetration".

In mid-1918, with the ending of the German spring offensive, the Australian troops started to conduct offensive patrols into no man's land. As the front lines after the Spring Offensive lacked fortifications and were non-continuous, it was discovered that the patrols could infiltrate the German outpost line and approach the outposts from behind. In this manner, the outposts could be taken quickly, and with minimal force. This tactic was first reported as being used on 5 April 1918 by the Australian 58th Battalion, 15th Brigade, 5th Division. However, within a few weeks all five of the Australian divisions were using the tactic, with some units using the tactic more than others (for example, the 3rd Division conducted the tactic on three out of every five days in April). In some units, it was treated as a competition, with companies of the 41st Battalion competing to see who could capture the most prisoners.

A similar tactic was used in Messines in 1917, referred to as "prospecting". Likewise, an earlier trench raid was made near Messines on 16 November 1915 by the Canadians.

The effect on German morale was quite pronounced, with the effect of peaceful penetration being noticed by both the Allies and Germans. The chief of staff of the German 2nd Army issued the following report on 13 July 1918:

During the last few days the Australians have succeeded in penetrating, or taking prisoner, single posts or piquets. They have gradually—sometimes even in daylight—succeeded in getting possession of a majority of the forward zone of a whole division. Troops must fight. They must not give way at every opportunity and seek to avoid fighting... The best way to make the enemy more careful in his attempt to drive us bit by bit out of the outpost line and forward zone is to do active reconnaissance... If the enemy can succeed in scoring a success without any special support by artillery or assistance from special troops, we must be in a position to do the same.

A captured German soldier is reported as saying:

You bloody Australians, when you are in the line you keep us on pins and needles; we never know when you are coming over.