Simplify the following text for a Grade 9 native English-speaking person.
Original: Ernest I , Duke of Saxe-Gotha ( 1601-75 ) had married Elisabeth Sophie , the only child of John Philip , Duke of Altenburg and Gotha ( 1597-1638 ) , the eldest son of Frederick William I. When Elisabeth Sophie 's cousin Frederick William III , Duke of Altenburg , died unmarried 1672 , the entire first Altenburg line went extinct in male line , opening a succession strife .
Simplified: Ernest I , Duke of Saxe-Gotha ( 1601-75 ) had married Elisabeth Sophie , the only child of John Philip , Duke of Altenburg ( 1597-1638 ) . Elisabeth Sophie 's cousin Frederick William III , Duke of Altenburg , died unmarried 1672 .