Complex sentence: By the time that Brussels and Rotterdam were liberated, it had become difficult for the 21st Army Group to be supplied adequately; indeed one corps (VIII Corps) had been withdrawn from active service in order to free its transport for general use.
Split into simpler sentences: By the time that Brussels and Rotterdam were liberated, it had become difficult for the 21st Army Group to be supplied adequately. Indeed one corps (VIII Corps) was withdrawn from active service to free its transport for general use.