That Earl, frequently in trouble with the law for his violent behaviour, had been among the closest allies of the disaffected Richard of York in the early 1450s, but a wedge was driven between York and the Courtenays when Bonville became a client of the leading Yorkist magnate Richard Neville, Earl of Warwick.
When the first major phase of the Wars of the Roses broke out in 1459, Earl Thomas remained loyal to Henry VI.
After the Yorkists seized power and captured King Henry in 1460, he joined other south-western aristocrats including Henry Beaufort, Duke of Somerset in raising a Lancastrian army, which went to join the forces being gathered by the queen, Margaret of Anjou, in northern England.
[6] He was attainted by Parliament in November of that year, depriving his heirs of the earldom of Devon, the barony of Courtenay and his estates.
[7] Courtenay married, at Coventry, Warwickshire, shortly after 9 September 1456, Mary of Anjou, illegitimate daughter of Charles, Count of Maine.