In the early days of the SS, recruits were to be German nationals who could prove their Aryan ancestry back to 1800, unmarried, and without a criminal record. Recruits had to be between the ages of 17 and 23, at least and of a certain height. Recruits were required to have perfect teeth and eyesight and provide a medical certificate. Once World War II commenced in Europe, the physical requirements were no longer strictly enforced, and essentially any recruit who could pass a basic medical exam was considered for Waffen-SS service. As the war went on, the SS ranks opened up to “people perceived to be of related stock”. A number of Danes, Dutch, Norwegians, Swedes and Finns volunteered to fight in the Waffen-SS under the command of German officers. After Germany invaded the Soviet Union in Operation Barbarossa, recruits from France, Spain, Belgium, Hungary and the Balkans signed on. From 1942 onwards, further units of non-Germanic recruits were formed. Often racially motivated, legions were formed of their men that were once enemies of Germany, from Bosnia, Croatia, Ukraine, and Russia. By 1944, the German military began conscripting Estonians and Latvians in an effort to replenish their losses. The foreigners who served in the Waffen-SS numbered "some 500,000", including those who were pressured into service or conscripted.