Complex sentence: On March 21 2003, Allied forces advanced across Southern Iraq and US marines captured the new port part of Umm Qasr, spending several more days in fighting through the old town part of Umm Qasr encountering fierce resistance.
Split into simpler sentences: On March 21,2003, allied forces advanced across Southern Iraq and US marines captured the new port area of Umm Qasr. They then spend several more days fighting through the old town part of Umm Qasr encountering fierce resistance.