Austria invaded and fought the Serbian army at the Battle of Cer and Battle of Kolubara beginning on 12 August. Over the next two weeks, Austrian attacks were thrown back with heavy losses, which marked the first major Allied victories of the war and dashed Austro-Hungarian hopes of a swift victory. As a result, Austria had to keep sizeable forces on the Serbian front, weakening its efforts against Russia. Serbia's defeat of the Austro-Hungarian invasion of 1914 has been called one of the major upset victories of the twentieth century. The campaign saw the first use of medical evacuation by the Serbian army in autumn of 1915 and anti-aircraft warfare in the spring of 1915 after an Austrian plane was shot down with ground-to-air fire.