Complex sentence: The unit was ashore at the former Clark Air Base (known today as the Diosdado Macapagal International Airport) participating in an exercise, when the typhoon hit and their mission changed from participation in the exercise to disaster relief.
Split into simpler sentences: The unit was ashore at the former Clark Air Base (known today as the Diosdado Macapagal International Airport) participating in an exercise when the typhoon hit. Their mission changed from participation in the exercise to disaster relief.