Modern armies move by air, by sea, and, on the ground, by track and wheel. Animal transport is such an anachronism that it hasn't been seen since Teddy Roosevelt went up San Juan (Kettle) hill.
This popular view of modern warfare, like most other popular views, is pretty much wrong. Most First and Second world armies are fully motorized today, but use of animals, even in the most powerful armies in the world, isn't that far removed in time. In some armies, animal transport is still commonly used even today.
Mechanization didn't solve all problems of transport and supply and reconnaissance until the advent of practical helicopters; and possibly not even then. Up through WW II, pack mules were still used in the United States Army in difficult terrain (primarily Italy).While the Polish Cavalry was destroyed in its attempt to defend the homeland, the Soviet Union learned enough to use large horse Cavalry formations effectively against the Germans. (albeit with heavy losses). The German Army, touted as the first truly mechanized army, moved the majority of its artillery and much of its tactical supply with draft animal. U.S. leaders also thought the horse could be effective in combat.
http://www.qmfound.com/horse.htm provides information on remount purchase and disposal, and quotes several leaders of the United States Army on the subject of horses and mules.
Lieutenant General L. K. Truscot, Jr. said; "The need for mounted reconnaissance and combat elements to work in close cooperation with the infantry in rough terrain was no less marked than the need for pack animals. The need for such elements is obvious. However, such elements cannot be improvised in combat from untrained personnel, although we made strenuous efforts to do so.
"I am firmly convinced that if one squadron of horse cavalry and one pack troop of 200 mules had been available to me at San Stefano on August 1, they would have enabled me to cut off and capture the entire German force opposing me along the north coast road and would have permitted my entry into Messina at least 48 hours earlier."
General George S. Patton stated; ""It is the considered opinion, not only of myself but of many other general officers who took their origin from the infantry and artillery, that had we possessed an American cavalry division with pack artillery in Tunisia and in Sicily, not a German would have escaped, because horse cavalry possesses the additional gear ratio which permits it to attain sufficient speed through mountainous country to get behind and hold the enemy until the more powerful infantry and tanks can come up and destroy him."
