General Mack thought that Austrian security relied on sealing off the gaps through the mountainous Black Forest area in Southern Germany that had witnessed much fighting during the campaigns of the French Revolutionary Wars. Mack believed that there would be no action in Central Germany. Mack decided to make the city of Ulm the centrepiece of his defensive strategy, which called for containment of the French until the Russians under Kutuzov could arrive and alter the odds against Napoleon. Ulm was protected by the heavily fortified Michelsberg heights, giving Mack the impression that the city was virtually impregnable from outside attack. Fatally, the Aulic Council decided to make Northern Italy the main theatre of operations for the Habsburgs. Archduke Charles was assigned 95,000 troops and directed to cross the Adige River with Mantua, Peschiera, and Milan as the initial objectives. Archduke John was given 23,000 troops and commanded to secure Tyrol while serving as a link between his brother, Charles, and his cousin, Ferdinand; the latter's force of 72,000, which was to invade Bavaria and hold the defensive line at Ulm, was effectively controlled by Mack. The Austrians also detached individual corps to serve with the Swedish in Pomerania and the British in Naples, though these were designed to obfuscate the French and divert their resources.