Prepared positions made enemy armies difficult to dislodge. Going around them was impractical when they controlled a river needed for supplies. A siege could take weeks or months and end in a costly assault, as Wellington's experience in the Peninsular War illustrates. Drawing an aggressive opponent into the field offered the possibility of defeating it much sooner. This is the proposed strategic reasoning behind Grant's willingness to leave his camp without extensive earthworks.
Notable Point · 1:08:03-1:10:19
Why draw an army out of its defences?
An enemy guarding a vital supply river could be difficult to bypass, while a siege consumed the time and troops a mobile campaign needed.