Complex sentence: The Red Sox challenged for the AL pennant in -49 (finishing second by a single game both seasons) thanks to Cronin's aggressive trades, but they began a slow decline during the 1950s and did not seriously contend after.
Split into simpler sentences: The Red Sox challenged for the AL pennant in -49 (finishing second by a single game both seasons) thanks to Cronin's aggressive trades. But the Red Sox began a slow decline during the 1950s and did not seriously contend after.