For many years the struggle dragged on. At times it seemed probable that Farnese and the Spaniards would overcome the North by force as they had obtained the South by diplomacy. But a variety of reasons explain the ultimate success of the Dutch. The nature of the country rendered ordinary campaigning very difficult--the network of canals constituted natural lines of defense and the cutting of the dikes might easily imperil an invading army. Again, the seafaring propensities of the Dutch stimulated them to fit out an increasing number of privateers which constantly preyed upon Spanish commerce: it was not long before this traffic grew important and legitimate, so that in the following century Amsterdam became one of the greatest cities of the world, and Holland assumed a prominent place among commercial and colonial nations. Thirdly, the employment of foreign mercenaries in the army of defense enabled the native population to devote the more time to peaceful pursuits, and, despite the persistence of war, the Dutch provinces increased steadily in wealth and prosperity. Fourthly, the cautious Fabian policy of William the Silent prevented the Dutch from staking heavily upon battles in the open field. Fifthly, the Dutch received a good deal of assistance from Protestants of Germany, England, and France. Finally, Philip II pursued too many great projects at once to be able to bring a single one to a satisfactory conclusion: his war with Queen Elizabeth of England and his interference in the affairs of France inextricably complicated his plans in the Netherlands. [Sidenote: Formal Declaration of Dutch Independence, 1581] In 1581 Philip II published a ban against William of Orange, proclaiming him a traitor and an outlaw and offering a reward to any one who would take him dead or alive. William replied by his famous "Apology" to the charges against him; but his practical answer to the king was the Act of Abjuration, by which at his persuasion the representatives of the northern provinces, assembled at The Hague, solemnly proclaimed their separation from the crown of Spain, broke the royal seal of Philip II, and declared the king deprived of all authority over them. We should call this Act of 1581 the Dutch declaration of independence. It was an augury of the definitive result of the war. [Sidenote: Recognition of Dutch Independence]