 Complete the following phrase:  The following campaign witnessed a change in Charles strategy. Realizing his numerical weakness he abandoned the idea of an envelopment, and decided to use Oxfordshire as the stronghold from which he could strike in all directions. The commanding situation of the city itself prevented any serious attempt at investment by dividing the enemys forces, but material wants made it impossible for Charles to maintain permanently his central position. Plans were continually resolved upon and cancelled on both sides, and eventually Essex headed for the south-west, leaving Waller to face the king alone. The 