Complex sentence: The kingdom reached its height under Áedán mac Gabráin (r.574--608), but King Æthelfrith of Bernicia checked its growth at the Battle of Degsastan in 603.
Split into simpler sentences: The kingdom reached its height under Áedán mac Gabráin (r.574--608). However, King Æthelfrith of Bernicia checked its growth at the Battle of Degsastan in 603.