When war erupted in Europe in August 1914, Britain quickly requested Japanese assistance under the terms of the Anglo-Japanese Alliance signed in 1902. On August 15, Japan issued an ultimatum to Germany: withdraw all warships from Chinese and Japanese waters and transfer control of Qingdao. Major-General Mitsuomi Kamio of the 18th Infantry Division was ordered to prepare for an assault the next day. The ultimatum expired on August 23, and Japan declared war. Meanwhile, Vice Admiral Maximilian von Spee dispersed the ships of the German East Asia Squadron from their Qingdao base to various Pacific destinations, eventually sailing to South America where they won the Battle of Coronel before being destroyed at the Battle of the Falkland Islands. The port was left to defend itself.

Germany had fortified Qingdao with three defensive lines. The outermost stretched along three hills named Mount Moltke, Mount Bismarck, and Mount Iltis, fortified with heavy guns: two 240mm weapons at Fort Moltke, four 280mm howitzers at Fort Bismarck, and two more 240mm guns at Fort Iltis. A network of trenches, batteries, and redoubts connected the positions. The harbor approaches were mined. The garrison, commanded by naval Captain and Governor Alfred Meyer-Waldeck, numbered 3,625 men: marines of the III Seebataillon, naval personnel, Chinese colonial troops, and Austro-Hungarian sailors from the Kaiserin Elisabeth. Against them came 23,000 Japanese soldiers supported by 142 artillery pieces, along with 1,500 British troops including the 2nd Battalion of the South Wales Borderers and 500 soldiers of the 36th Sikhs. After a friendly fire incident, British troops were given Japanese raincoats to avoid being shot by their allies.

The Japanese began landing on September 2 at Lungkow, experiencing heavy floods, and later at Lau Schan Bay. Meyer-Waldeck withdrew to his innermost defensive line as the siege tightened. On September 27, the Japanese attempted a frontal assault on Prince Heinrich Hill and were caught in devastating crossfire from four Maxim guns on the summit while the Kaiserin Elisabeth and the gunboat Leopard shelled their exposed flanks from the harbor. The assault nearly routed the Japanese right flank before the allied fleet intervened. The Kaiserin Elisabeth's crew eventually removed her 15cm and 4.7cm guns and mounted them ashore as the Batterie Elisabeth, fighting alongside the infantry. The colony surrendered on November 7, 1914, after a defense that Kaiser Wilhelm II had personally demanded be maintained to the last.

The siege's consequences far outlasted the fighting. Japan occupied Qingdao until 1922, and at the Paris Peace Conference in 1919, the Allies awarded former German Pacific territories to Japan rather than returning them to China. This decision triggered the May Fourth Movement, a watershed in Chinese political consciousness that helped give rise to the Chinese Communist Party. The British soldiers who fought at Tsingtao did not linger in Asia. The 2nd Battalion South Wales Borderers embarked for England in November 1914, arriving in January 1915. They joined the newly formed 29th Division and were sent to Gallipoli in April 1915, trading one distant siege for another. The Siege of Tsingtao was a small battle by the standards of the Western Front, but its political reverberations reshaped Asia for the rest of the century.