Complex sentence: Upon approaching Earth, the Scrin are surprised to encounter a heavily militarized native population still coping with Tiberium's spread, and swiftly proceed to launch an assault on Earth's cities, wreaking havoc on GDI and Nod alike.
Split into simpler sentences: Upon approaching Earth, the Scrin are surprised to encounter a heavily militarized native population. The Scrin swiftly proceeded to launch an assault on Earth's cities, wreaking havoc on GDI and Nod alike.