Complex sentence: Upon hearing that Linus had possession of "his" most prized possession, Snoopy gave Linus back the blanket fairly quickly, thinking, "I never dreamed he would fight so dirty!".
Split into simpler sentences: Upon hearing that Linus had possession of "his" most prized possession, he gave Linus back the blanket fairly quickly. His thought on that was "I never dreamed he would fight so dirty!".