As soon as Charles departed for his expedition to Algiers, the Netherlands were again exposed to the attacks of his enemies, including Francis I, the King of Denmark and the Duke of Cleves, who had inherited the county of Gelder. This time Mary was strongly supported by the States General, and succeeded in facing the attacks on both sides pending the return of the emperor (1543). The latter took the opportunity given him by a prompt victory to settle once for all the Gelder question by the treaty of Venloo. The Duke of Cleves was obliged to renounce all rights over Gelder and Zutphen, which became integral parts of the Netherlands. This was the last act of the work of territorial unification pursued by the dukes of Burgundy. At the same time, in order to protect the Low Countries from French attacks, Charles V fortified the three towns of Marienbourg, Charlemont and Philippeville, called after Mary of Hungary, Charles himself and his son Philip.
Thus, at last, the Low Countries reaped some advantage from the constant expenses which they had to sustain owing to incessant European wars. They were no longer able to pursue an independent policy, and, if the States preserved a certain liberty, it was mainly because they could be induced to vote war-taxes, these being, so to speak, the ransom which the so-called "free" Netherlands paid to their ruler. During Charles's youth, almost all the revenues of the State had been drawn from the prince's domain, but towards the end of his reign the levies extorted from the people became more and more heavy and frequent. The annual budget rose from one million pounds in 1541 to two and a half millions in 1542 and six and a half millions in 1555. To these annual contributions we must add the numerous loans raised by the Government on the security of the provinces. The interest on these loans weighed heavily on the budget. It was £141,300 in 1552, £424,765 in 1555, and rose to £1,357,287 in 1556. As a matter of fact, the States General could grant taxes but not control expenditure, so that most of the money raised in the Netherlands was spent on foreign expeditions from which the country could reap no benefit. Up to 1552, when gold from Mexico and Peru arrived in Spain, the Low Countries remained the main source of the income of the emperor.