At the start of World War I, Albert refused to comply with Germany's request for safe passage for its troops through Belgium in order to attack France, which the Germans alleged was about to advance into Belgium en route to attacking Germany in support of Russia. In fact, the French government had told its army commander not to go into Belgium before a German invasion. The German invasion brought Britain into the war as one of the guarantors of Belgian neutrality under the Treaty of 1839. King Albert, as prescribed by the Belgian constitution, took personal command of the Belgian Army, and held the Germans off long enough for Britain and France to prepare for the Battle of the Marne (6–9 September 1914). He led his army through the Siege of Antwerp (28 September – 10 October 1914) and the Battle of the Yser (16–31 October 1914), when the Belgian Army was driven back to a last, tiny strip of Belgian territory near the North Sea. Here the Belgians, in collaboration with the armies of the Triple Entente, took up a war of position, in the trenches behind the River Yser, remaining there for the next four years. During this period, King Albert fought alongside his troops and shared their dangers, while his wife, Queen Elisabeth, worked as a nurse at the front. During his time on the front, rumours spread on both sides of the lines that the German soldiers never fired upon him out of respect for him being the highest ranked commander in harm's way, while others feared risking punishment by the Kaiser himself, who was his cousin. The King also allowed his 12-year-old son, Prince Leopold, to enlist in the Belgian Army as a private and fight in the ranks. In the final offensive of the war, he commanded the at the Fifth Battle of Ypres.