Complex sentence: After the failure of the first invasion, Darius began raising a huge new army with which he meant to subjugate Greece completely; however, in 486 BC, his Egyptian subjects revolted, indefinitely postponing any Greek expedition.
Split into simpler sentences: After the failure of the first invasion, Darius began raising a huge new army with which he intended to subjugate Greece completely. However, in 486 BC, his Egyptian subjects revolted, and the revolt forced an indefinite postponement of any Greek expedition.