Alexander the Great inherited one of the most powerful armies in the ancient world from his father Philip. The army used a unique spear phalanx that was virtually unheard of outside of Macedonia, and the army was well trained and supported. Additionally Philip felt the use of siege weaponry was of great importance and invested in engineers to aid him. Philip even used siege weaponry on the field in battle some times. Philip set the stage for his son to dominate the known world, and even gave him a road map for conquering Asia minor, but an army and siege weapons are only a portion of an armies success. Especially when planning a long protracted campaign far from home.
Alexander’s conquest of Asia was heavily influenced by two very important factors. The ability to logistically supply his army across vast areas and the ability of his engineers to allow him to take cities without long protracted sieges. This allowed Alexander to quickly expand his empire without having to worry about opponents being able to exploit his absence in other areas. Perhaps even more important than siege tactics and improved weaponry were the logistics that allowed Alexander to continue with his campaign without running out of food, water, and fodder for his army. An army is limited in how much food and water can be carried by personnel and animals and must forage or have some logistical support to supply it over a long period.
An average ration for a grown man on campaign with a full load would have been three pounds of grain or something else similarly nutritional and two quarts of water. The animals also needed to be fed and an average for a horse was ten pounds of food and eight gallons of water.1 Alexander was known to disdain wagons and endeavored to reduce his supply train as much as possible by reducing the number of camp followers. When they left the army took a very small train with them when leaving Macedonia. Only enough carts and animals to carry the siege equipment, tents, rations, and other equipment required by the army were used. All additional rations would have to have been carried via Alexander’s fleet. Memnon wanted to pursue a policy of scorched earth and then bring the war to Greece to prevent Alexander from being able to wage war abroad, and if the other Persian generals had followed his advice Alexander would have most likely had to return to Macedonia and never been able to accomplish anything outside of Greece. Due to the Persian's vanity they decided to instead attack Alexander directly without razing the countryside and were defeated leading to Alexander gaining the foothold he needed in Asia.
A likely scenario would put a seven day limit on the amount of rations the army carried with it in a temperate climate and a two day limit in desert regions where the army could not forage for additional food, water, and fodder. No matter how many pack animals an army used the maximum amount of rations that could be carried would be twenty five...