Meanwhile, the Imperial forces had surged in Germany after the initial setback from the intervention of Christian IV of Denmark in the war in 1625. Both the Danes and Mansfelt were defeated in 1626, and the Catholic League occupied the northern German lands that had hitherto acted as a buffer zone for the Republic. For a while in 1628 an invasion of the eastern part of the Republic seemed imminent. However, the relative might of Spain, the main player up to now in the German civil war, was ebbing fast. By April 1629 the States Army counted 77,000 soldiers, half as much again as the Army of Flanders at that point in time. This allowed Frederick Henry to raise a mobile army of 28,000  and invest 's-Hertogenbosch. During the siege of this strategic fortress city the imperialist and Spanish allies launched a diversionary attack from Germany's IJssel line. After crossing this river, they invaded the Dutch heartland, getting as far as the city of Amersfoort, which promptly surrendered. The States General, however, mobilised civic militias and scrounged garrison troops from fortresses all around the country, assembling an army that at the height of the emergency numbered no less than 128,000 troops. This enabled Frederick Henry to maintain his siege of 's-Hertogenbosch. When Dutch troops surprised the Spanish fortress of Wesel, which acted as the principal Spanish supply base, this forced the invaders to retreat to the IJssel. 's-Hertogenbosch surrendered in September 1629 to Frederick Henry.. How many months did the invasion of the Dutch last?