Complex sentence: After Robert dies, Renly declares himself King of the Seven Kingdoms in "A Clash of Kings", wins the support of the Baratheon and Tyrell bannermen, and seals an alliance with House Tyrell by marrying Margaery Tyrell.
Split into simpler sentences: After Robert dies, Renly declares himself King of the Seven Kingdoms in "A Clash of Kings" and wins the support of the Baratheon and Tyrell bannerman. He also seals an alliance with House Tyrell by marrying Margaery Tyrell.