Complex sentence: Arsenal represented England in the European Cup for the first time since the ban on English teams from playing in European club competitions, but their time in the competition was short-lived as Portuguese team Benfica eliminated them in the second round.
Split into simpler sentences: Arsenal represented England in the European Cup; they were the first club to do so since the ban on English teams from playing in European club competitions. Their time in the competition was brief however as Portuguese team Benfica eliminated them in the second round.