Infanta Isabella Clara Eugenia (1566-1633) was the eldest daughter of Philip II of Spain and his third wife Elisabeth de Valois. In 1599, she was married to her cousin Albert VII, Archduke of Austria (1559-1621), son of Holy Roman Emperor Maximilian II and Maria of Spain. As she had agreed with her father, Isabella & her husband also became rulers of the Hapsburg Netherlands from that moment on.
OTL, Albert and Isabella left no children behind and, as a result, the Hapsburg Netherlands were returned to Spain (being inherited by Isabella's nephew, King Philip IV). However, what would be the consequences of a son being born to Isabella & Albert?