Romero fought in the Siege of Mons (1572), where he nearly succeeded in killing William the Silent in a daring raid against the Dutch camp. He was also present at the Massacre of Naarden and the Siege of Haarlem, where he lost an eye. In 1574, he failed to relieve Middelburg after losing the Battle of Reimerswaal, and in 1576 he was present at the Sack of Antwerp.