Germany sent only a small contingent to the relief of the foreign legions in Peking, but Kaiser Wilhelm II sent his men with this command: "Bear yourselves as Huns of Attila. For a thousand years, let the Chinese tremble at the approach of a German." The German imperial troops obeyed, with so much rape, looting and murder of Chinese citizens that the American and (ironically, given the events of the next 45 years) Japanese troops had to turn their guns several times on the Germans and threaten to shoot them, to restore order.
Wilhelm and his army were motivated most immediately by the murder of the two German missionaries in Shandong Province. However, their larger motivation was that Germany had only unified in as a nation in 1871. The Germans felt that they had fallen behind European powers like the United Kingdom and France, and Germany wanted its own "place in the sun" - its own empire. Collectively, they were prepared to be utterly ruthless in pursuit of that goal.
The Battle of Tientsin would be the bloodiest of the Boxer Rebellion. In an unsettling preview of World War I, the foreign troops ran across open ground to attack the fortified Chinese positions and were simply mown down; the Chinese regulars on the city walls had Maxim guns, an early machine-gun, as well as cannon. Foreign casualties at Tientsin topped 750.