THE Anatomy Of The US Army Haversack - WWI / WWII - The haversack was the primary means for US soldiers to carry their food, supplies and general belongings during WWI and the early days of WWII.  The precursor to the backpack, the haversack proved to be too cumbersome for use in battle.  The items that went into the pack were not easily accessible.  You pretty much had to take the whole thing apart in order to the personal belongings.

Another flaw with the design is that the pack attached directly to the utility belt.  If a soldier needed to go in a light  mission he had to carry the entire pack - or leave everything behind. 

As the war progressed, different styles of backpacks were developed.  Including the precursor to the modern ALICE pack.

This page provides a comprehensive breakdown of how the haversack was designed and how the rest of the soldier's equipment connected to it.