Complex sentence: When most of the Guardians'most powerful members were sent into space to thwart a Martian invasion, they were successful; but while they were away, the team's weaker members were required to stop the Lizard League's plot to take command of nuclear warheads.
Split into simpler sentences: When most of the Guardians'most powerful members were sent into space to thwart a Martian invasion. They were successful, but while they were away, the team's weaker members were required to stop the Lizard League's plot to take command of nuclear warheads.