Complex sentence: D.C. United entered 2015 following the team's remarkable 2014 campaign, during which the team registered the greatest turnaround from one MLS season to the next and also saw the team go undefeated in 2014-15 CONCACAF Champions League group play.
Split into simpler sentences: D.C. United entered 2015 with fresh memories of the team's remarkable 2014 campaign, during which the team registered the greatest turnaround in the history of Major League Soccer from one MLS season to the next. The 2014 campaign also saw D.C. United go undefeated in 2014-15 CONCACAF Champions League group play.