Complex sentence: Direct assault was forbidden against the holdout forts in order to preserve artillery ammunition for the XX Corps' advance to the Sarre River and the isolated forts subsequently surrendered one by one following the surrender of Fort Verdun on November 26.
Split into simpler sentences: Direct assault was forbidden against the holdout forts in order to preserve artillery ammunition for the XX Corps' advance to the Sarre River. However, the isolated forts subsequently surrendered one by one following the surrender of Fort Verdun on November 26.