Civil disturbances and religious persecutions were not the only causes of anxiety to the political leaders in the United Provinces during the crisis of 1618-19; foreign affairs were also assuming a menacing aspect. The year 1618 saw the opening in Germany of the Thirty Years' War. The acceptance of the Crown of Bohemia by Frederick, Elector Palatine, meant that the long-delayed struggle for supremacy between Catholics and Protestants was to be fought out; and it was a struggle which neither Spain nor the Netherlands could watch with indifference. Maurice was fully alive to the necessity of strengthening the defences of the eastern frontier; and subsidies were granted by the States-General to Frederick and also to some of the smaller German princes. This support would have been larger, but the unexpected refusal of James I to give aid to his son-in-law made the Dutch doubtful in their attitude. The States, though friendly, were unwilling to commit themselves. In the spring of 1620, however, by James' permission, the English regiments in the Dutch service under the command of Sir Horace Vere were sent to oppose Spinola's invasion of the Rhineland. Accompanied by a Dutch force under Frederick Henry, they reached the Palatinate, but it was too late. The fate of the King of Bohemia was soon to be decided elsewhere than in his hereditary dominions. Completely defeated at the battle of Prague, Frederick with his wife and family fled to Holland to seek the protection of their cousin, the Prince of Orange. They met with the most generous treatment at his hands, and they were for many years to make the Hague the home of their exile.

As the date at which the Twelve Years' Truce came to an end drew near, some efforts were made to avert war. There were advocates of peace in the United Provinces, especially in Gelderland and Overyssel, the two provinces most exposed to invasion.

The archdukes had no desire to re-open hostilities; and Pecquinius, the Chancellor of Brabant, was sent to the Hague to confer with Maurice, and was authorised to name certain conditions for the conclusion of a peace. These conditions proved, however, to be wholly unacceptable, and the early summer of 1621 saw Maurice and Spinola once more in the field at the head of rival armies. The operations were, however, dilatory and inconclusive. The stadholder now, and throughout his last campaigns, was no longer physically the same man as in the days when his skilful generalship had saved the Dutch republic from overthrow; he had lost the brilliant energy of youth. The deaths in the course of this same year, 1621, of both the Archduke Albert and Philip III of Spain, were also hindrances to the vigorous prosecution of the war. In 1622 there was much marching and counter-marching, and Maurice was successful in compelling Spinola to raise the siege of Bergen-op-Zoom, the last success he was destined to achieve. In the course of this year the prince's life was in serious danger. A plot was laid to assassinate him on his way to Ryswyck, the leading conspirator being William van Stoutenberg, the younger son of Oldenbarneveldt. Stoutenberg had, in 1619, been deprived of his posts and his property confiscated, and he wished to avenge his father's death and his own injuries. The plot was discovered, but Stoutenberg managed to escape and took service under the Archduchess Isabel. Unfortunately he had implicated his elder brother, Regnier, lord of Groeneveldt, in the scheme. Groeneveldt was seized and brought to the scaffold.

From this time nothing but misfortune dogged the steps of Maurice, whose health began to give way under the fatigues of campaigning. In 1623 a carefully planned expedition against Antwerp, which he confidently expected to succeed, was frustrated by a long continuance of stormy weather. Spinola in the following year laid siege to Breda. This strongly fortified town, an ancestral domain of the Princes of Orange, had a garrison of 7000 men. The Spanish commander rapidly advancing completely invested it. Maurice, who had been conducting operations on the eastern frontier, now hastened to Breda, and did his utmost by cutting off Spinola's own supplies to compel him to raise the blockade. All his efforts however failed, and after holding out for many months Breda surrendered. In the spring of 1625 the prince became so seriously ill that he asked the States-General to appoint his brother commander-in-chief in his stead. Feeling his end drawing near, Maurice's chief wish was to see Frederick Henry married before his death. Frederick Henry, like Maurice himself, had never shown any inclination for wedlock and there was no heir to the family. He had, however, been attracted by the Countess Amalia von Solms, a lady of the suite of Elizabeth of Bohemia. Under pressure from the dying man the preliminaries were speedily arranged, and the wedding was quietly celebrated on April 4. Though thus hastily concluded, the marriage proved to be in every way a thoroughly happy one. Amalia was throughout his life to be the wise adviser of her husband and to exercise no small influence in the conduct of public affairs. Maurice died on April 23, in the fifty-eighth year of his age. His forty years of continuous and strenuous service to the State had made him prematurely old; and there can be but little doubt that the terrible anxieties of the crisis of 1618-19 told upon him. Above all a feeling of remorse for his share in the tragedy of Oldenbarneveldt's death preyed upon his mind.