In 1632 the Prince of Orange, invading unhindered, had seized Venloo, Roermond, and last of all the great fortress of Maestricht. A more daring and less perceptive leader would have marched on Brussels. Frederick Henry was held back by two considerations: in the first place he was uncertain whether his army would be strong enough to hold the line of communications between the frontier and the Flemish capital, and in the second neither he nor the government of the United Provinces was altogether certain that the fall of Brussels was desirable. There had been a secret agreement with Rechelieu to split the Spanish Netherlands from end to end, France absorbing the southern and the Dutch the northern half. But Frederick Henry saw that the destruction of Hapsburg power was leading already to the unbalanced aggrandisement of the Bourbon, and was henceforward determined to preserve at all costs the bufer between himself and the rising monarchy. Unknown to the Brussel government, the Dutch, their open enemies, were becoming the guarantors of their existence against the aggression of France.