The Passage of Lines

An Essential Battlefield Skill in the Ukrainian Campaigns Ahead

1st (UK) Armoured Division in its assembly area in 1991 before conducting a passage of lines through the 1st (US) Infantry Division (Image: TankMuseum.org)

For ground forces, the operation to replace one unit with another, with the intent of moving into or out of contact with an enemy, is known as a Passage of Lines.  This is a form of operation that is particularly relevant in observing the war in Ukraine because we are likely to see both the Ukrainians and the Russians undertake this difficult type of manoeuvre in the coming weeks.

In military operations, units and formations often have to be replaced. There are multiple reasons this might occur in either defensive or offensive operations. Units might have been in combat for an extended period, require rest and reinforcement and there are fresh units to replace them. In other circumstances, a unit undertaking one mission might be required for a different mission elsewhere and must be replaced. Or, finally, to sustain the tempo of an advance, units might be replaced with fresher or more mobile organisations.

The US Army doctrine on this topic defines a passage of lines as “an operation in which a force moves forward or rearward through another force’s combat positions with the intention of moving into or out of contact with the enemy.” The inclusion of forward or rearward passage of lines is important. Both necessary capabilities for professional and effective ground combat forces, and they are likely to feature in the near-future operations conducted in Ukraine.

The conduct of a passage of lines, while in contact with the enemy, is a very complex and difficult operation to undertake successfully. Coordination measures between units or formations are vital, as are issues such as air defence, logistics and operational security. Additionally, speed of action is important as is the use of night, smoke and electronic measures to screen the conduct of units replacing each other.