Habsburg Austria and Bourbon France, traditional enemies, allied together in the Diplomatic Revolution of 1756 following the collapse of the Anglo-Austrian Alliance. Frederick swiftly made an alliance with Great Britain at the Convention of Westminster. As neighboring countries began conspiring against him, Frederick was determined to strike first. On 29 August 1756 his well-prepared army crossed the frontier and invaded Saxony, thus beginning the Seven Years' War which lasted until 1763. He faced widespread criticism for his attack on neutral Saxony and for his forcible incorporation of the Saxon forces into the Prussian army following the Siege of Pirna in October 1756.