In March 1635, a French force entered the Valtellina, once again cutting the link between Milan and the Empire. In May, their main army of 35,000 invaded the Spanish Netherlands but was forced to retreat in July after suffering 17,000 casualties from disease and desertion. A Spanish offensive in 1636 reached Corbie in Northern France; although it caused panic in Paris, lack of supplies forced them to retreat, and it was not repeated. In the March 1636 Treaty of Wismar, France formally joined the Thirty Years War in alliance with Sweden; a Swedish army under Johan Banér entered Brandenburg and re-established their position in North-East Germany at Wittstock on 4 October 1636.