The cost proved extremely high, increased after 1628 by a proxy war with France over the Mantuan succession. While the Spanish Empire reached its maximum nominal extent under Philip's rule, its size and complexity made it increasingly difficult to govern, or enact essential reforms. Despite this, depth of resources consistently allowed them to recover from defeats that would have shattered other powers, while new regulations passed in 1631 and 1632 were key to improved Spanish military performance in the first part of the war. In 1628, the Dutch captured the Spanish treasure fleet, which they used to finance the capture of 's-Hertogenbosch the following year. The powerful Amsterdam mercantile lobby saw this as an opportunity to end the war; negotiations ended without result in 1633, but strengthened the peace party. The Peace of Prague led to rumours of a proposed Austro-Spanish offensive in the Netherlands, leading Louis XIII of France and Richelieu to decide on direct intervention. In early 1635, they signed an agreement with Bernard of Saxe-Weimar to provide 16,000 troops for a campaign in Alsace and the Rhineland, forming an anti-Spanish alliance with the Dutch, as well as signing the Treaty of Compiègne with Sweden. Phase I: 1635 to the 1648, Treaty of Westphalia In May, a French army of 27,000 invaded the Spanish Netherlands and defeated a smaller Spanish force at Les Avins, then besieged Leuven on 24 June, where they were joined by Dutch reinforcements. Disease and lack of supplies quickly reduced the besieging army, which withdrew in the face of a relief force under Ottavio Piccolomini on 4 July. Led by Cardinal-Infante Ferdinand of Austria, the Spanish took the initiative and captured Limbourg, Gennep, Diest, Goch, then besieged Dutch garrisons in the Duchy of Cleves. The French retreated across the border while the Dutch under Frederick Henry marched urgently on the strategic position of Schenkenschans. Captured by the Spanish on 28 July, it was recovered only after a long and costly siege. Following this failure, the States General of the Netherlands opposed further large scale land operations in favour of attacks on Spanish trade. In the campaign of 1636, Philip switched his focus to recovering territories in the Low Countries, while a Franco-Savoyard offensive in Lombardy was defeated at Tornavento in June. A Spanish incursion into northern France captured the key fortified town of Corbie in August but despite causing panic in Paris, lack of