Frederick Henry was at this period in his forty-second year. His military reputation was well established; he soon proved his political talents. He commenced his career by a total change in the tone of government on the subject of sectarian differences. He exercised several acts of clemency in favor of the imprisoned and exiled Arminians, at the same time that he upheld the dominant religion. By these measures he conciliated all parties; and by degrees the fierce spirit of intolerance became subdued. The foreign relations of the United Provinces now presented the anomalous policy of a fleet furnished by the French king, manned by rigid Calvinists, and commanded by a grandson of Admiral Coligny, for the purpose of combating the remainder of the French Huguenots, whom they considered as brothers in religion, though political foes; and during the joint expedition which was undertaken by the allied French and Dutch troops against Rochelle, the stronghold of Protestantism, the preachers of Holland put up prayers for the protection of those whom their army was marching to destroy. The states-general, ashamed of this unpopular union, recalled their fleet, after some severe fighting with that of the Huguenots. Cardinal Richelieu and the king of France were for a time furious in their displeasure; but interests of state overpowered individual resentments, and no rupture took place.

Charles I. had now succeeded his father on the English throne. He renewed the treaty with the republic, which furnished him with twenty ships to assist his own formidable fleet in his war against Spain. Frederick Henry had, soon after his succession to the chief command, commenced an active course of martial operations, and was successful in almost all his enterprises. He took Groll and several other towns; and it was hoped that his successes would have been pushed forward upon a wider field of action against the imperial arms; but the States prudently resolved to act on the defensive by land, choosing the sea for the theatre of their more active operations. All the hopes of a powerful confederation against the emperor and the king of Spain seemed frustrated by the war which now broke out between France and England. The states-general contrived by great prudence to maintain a strict neutrality in this quarrel. They even succeeded in mediating a peace between the rival powers, which was concluded the following year; and in the meantime they obtained a more astonishing and important series of triumphs against the Spanish fleets than had yet been witnessed in naval conflicts.

The West India Company had confided the command of their fleet to Peter Hein, a most intrepid and intelligent sailor, who proved his own merits, and the sagacity of his employers on many occasions, two of them of an extraordinary nature. In 1627, he defeated a fleet of twenty-six vessels, with a much inferior force. In the following year, he had the still more brilliant good fortune, near Havana, in the island of Cuba, in an engagement with the great Spanish armament, called the Money Fleet, to indicate the immense wealth which it contained. The booty was safely carried to Amsterdam, and the whole of the treasure, in money, precious stones, indigo, etc., was estimated at the value of twelve million florins. This was indeed a victory worth gaining, won almost without bloodshed, and raising the republic far above the manifold difficulties by which it had been embarrassed. Hein perished in the following year, in a combat with some of the pirates of Dunkirk—those terrible freebooters whose name was a watchword of terror during the whole continuance of the war.

The year 1629 brought three formidable armies at once to the frontiers of the republic, and caused a general dismay all through the United Provinces; but the immense treasures taken from the Spaniards enabled them to make preparations suitable to the danger; and Frederick Henry, supported by his cousin William of Nassau, his natural brother Justin, and other brave and experienced officers, defeated every effort of the enemy. He took many towns in rapid succession; and finally forced the Spaniards to abandon all notion of invading the territories of the republic. Deprived of the powerful talents of Spinola, who was called to command the Spanish troops in Italy, the armies of the archduchess, under the count of Berg, were not able to cope with the genius of the Prince of Orange. The consequence was the renewal of negotiations for a second truce. But these were received on the part of the republic with a burst of opposition. All parties seemed decided on that point; and every interest, however opposed on minor questions, combined to give a positive negative on this.

The gratitude of the country for the services of Frederick Henry induced the provinces of which he was stadtholder to grant the reversion in this title to his son, a child of three years old; and this dignity had every chance of becoming as absolute, as it was now pronounced almost hereditary, by the means of an army of one hundred and twenty thousand men devoted to their chief. However, few military occurrences took place, the sea being still chosen as the element best suited to the present enterprises of the republic. In the widely-distant settlements of Brazil and Batavia, the Dutch were equally successful; and the East and West India companies acquired eminent power and increasing solidity.

The year 1631 was signalized by an expedition into Flanders, consisting of eighteen thousand men, intended against Dunkirk, but hastily abandoned, in spite of every probability of success, by the commissioners of the states-general, who accompanied the army, and thwarted all the ardor and vigor of the Prince of Orange. But another great naval victory in the narrow seas of Zealand recompensed the disappointments of this inglorious affair.

The splendid victories of Augustus Adolphus against the imperial arms in Germany changed the whole face of European affairs. Protestantism began once more to raise its head; and the important conquests by Frederick Henry of almost all the strong places on the Meuse, including Maestricht, the strongest of all, gave the United Provinces their ample share in the glories of the war. The death of the archduchess Isabella, which took place at Brussels in the year 1633, added considerably to the difficulties of Spain in the Belgian provinces. The defection of the count of Berg, the chief general of their armies, who was actuated by resentment on the appointment of the marquis of St. Croix over his head, threw everything into confusion, in exposing a widespread confederacy among the nobility of these provinces to erect themselves into an independent republic, strengthened by a perpetual alliance with the United Provinces against the power of Spain. But the plot failed, chiefly, it is said, by the imprudence of the king of England, who let the secret slip, from some motives vaguely hinted at, but never sufficiently explained. After the death of Isabella, the prince of Brabancon was arrested. The prince of Epinoi and the duke of Burnonville made their escape; and the duke of Arschot, who was arrested in Spain, was soon liberated, in consideration of some discoveries into the nature of the plot. An armistice, published in 1634, threw this whole affair into complete oblivion.

The king of Spain appointed his brother Ferdinand, a cardinal and archbishop of Toledo, to the dignity of governor-general of the Netherlands. He repaired to Germany at the head of seventeen thousand men, and bore his share in the victory of Nordlingen; after which he hastened to the Netherlands, and made his entry into Brussels in 1634. Richelieu had hitherto only combated the house of Austria in these countries by negotiation and intrigue; but he now entered warmly into the proposals made by Holland for a treaty offensive and defensive between Louis XIII. and the republic. By a treaty soon after concluded (February 8, 1635) the king of France engaged to invade the Belgian provinces with an army of thirty thousand men, in concert with a Dutch force of equal number. It was agreed that if Belgium would consent to break from the Spanish yoke it was to be erected into a free state; if, on the contrary, it would not co-operate for its own freedom, France and Holland were to dismember, and to divide it equally.

The plan of these combined measures was soon acted on. The French army took the field under the command of the marshals De Chatillon and De Breeze; and defeated the Spaniards in a bloody battle, near Avein, in the province of Luxemburg, on the 20th of May, 1635, with the loss of four thousand men. The victors soon made a junction with the Prince of Orange; and the towns of Tirlemont, St. Trond, and some others, were quickly reduced. The former of these places was taken by assault, and pillaged with circumstances of cruelty that recall the horrors of the early transactions of the war. The Prince of Orange was forced to punish severely the authors of these offences. The consequences of this event were highly injurious to the allies. A spirit of fierce resistance was excited throughout the invaded provinces. Louvain set the first example. The citizens and students took arms for its defence; and the combined forces of France and Holland were repulsed, and forced by want of supplies to abandon the siege, and rapidly retreat. The prince-cardinal, as Ferdinand was called, took advantage of this reverse to press the retiring French; recovered several towns; and gained all the advantages as well as glory of the campaign. The remains of the French army, reduced by continual combats, and still more by sickness, finally embarked at Rotterdam, to return to France in the ensuing spring, a sad contrast to its brilliant appearance at the commencement of the campaign.

The military events for several ensuing years present nothing of sufficient interest to induce us to record them in detail. A perpetual succession of sieges and skirmishes afford a monotonous picture of isolated courage and skill; but we see none of those great conflicts which bring out the genius of opposing generals, and show war in its grand results, as the decisive means of enslaving or emancipating mankind. The prince-cardinal, one of the many who on this bloody theatre displayed consummate military talents, incessantly employed himself in incursions into the bordering provinces of France, ravaged Picardy, and filled Paris with fear and trembling. He, however, reaped no new laurels when he came into contact with Frederick Henry, who, on almost every occasion, particularly that of the siege of Breda, in 1637, carried his object in spite of all opposition. The triumphs of war were balanced; but Spain and the Belgian provinces, so long upheld by the talent of the governor-general, were gradually become exhausted. The revolution in Portugal, and the succession of the duke of Braganza, under the title of John IV., to the throne of his ancestors, struck a fatal blow to the power of Spain. A strict alliance was concluded between the new monarch of France and Holland; and hostilities against the common enemy were on all sides vigorously continued.