Occurring at the mid-point in Napoleon’s imperial career, the Franco-Austrian War of 1809, or the War of the Fifth Coalition, highlighted the strengths and weaknesses of the French emperor and his army as well as the beginnings of improvements among their foes. Austria, driven by a desire to avenge previous defeats and hoping to take advantage of Napoleon’s distraction in Spain, opened hostilities by invading Napoleon’s ally Bavaria, but the French emperor hastened to the theater of war, quickly seized the initiative and entered Vienna only one month after departing Paris. The ensuing conflict was fought across a vast geographic canvas. Combat in the principal theater, the Danube valley, featured Napoleon’s first undeniable repulse at Aspern-Essling (21–22 May), the second largest battle of the entire epoch at Wagram (5–6 July) and a surprisingly sudden armistice at Znaim six days later, but the war also encompassed strategically important actions in subsidiary theaters such as Italy, Poland, Hungary, Germany and Holland. It led to Austrian accommodation with France, Napoleon’s marriage to a Habsburg archduchess and eventually to a Franco-Austrian alliance, but it also deepened Franco-Russian suspicions and thus helped set the stage for war in 1812