He found his course blocked by a Spanish force under the command of Sancho d'Avila and Mondragon.
The encounter took place on the heath of Mook (April 14) and ended in the crushing defeat of the invaders.

Lewis and his young brother, Henry, and Duke Christopher perished, and their army was completely scattered.

The death of his brothers was a great grief to William.
Lewis had for years been his chief support, and the loss of this dauntless champion was indeed a heavy blow to the cause for which he had sacrificed his life.

He was only thirty-six years of age, while Henry, the youngest of the Nassaus, to whom the Prince was deeply attached, was but a youth of twenty-four.
The invasion of Lewis had nevertheless the result of raising the siege of Leyden; but only for a time.

After the victory at Mook the Spanish troops were free to continue the task of reconquering rebel Holland for the king.