By 1743, the Austrians had subdued Bavaria and driven the French out of Bohemia. Frederick strongly suspected Maria Theresa would resume war in an attempt to recover Silesia. Accordingly, he renewed his alliance with France and preemptively invaded Bohemia in August 1744, beginning the Second Silesian War. By late August 1744, all of Frederick's columns had crossed the Bohemian frontier. Frederick marched straight for Prague and laid siege to the city. On 11 September 1744, the Prussians began a three-day artillery bombardment of Prague, which fell a few days later. Three days after this victory, Frederick's troops were again on the march into the heart of central Bohemia. However, the Austrians refused to directly engage with Frederick's army and harassed his supply lines, eventually forcing him to withdraw to Silesia as winter approached. With the death of Holy Roman Emperor Charles VII of Bavaria in January 1745, Maria Theresa's husband Francis of Lorraine was elected emperor and Saxony joined the Austrians' side against Frederick.