Invasion of Belgium. On July 27 Belgium and Luxembourg were informed by Germany of its intention to pass through. Both countries were assured that no harm would come to persons or property if no opposition were encountered.

Belgian neutrality had been guaranteed by all the European powers in the Treaty of London in 1839. Despite this agreement. German Chancellor Theobald von Bethmann-Hollweg tried to persuade Britain not to declare war in the event that German troops entered Belgium. During talks on August 3 with Sir Edward Goschen. the British ambassador to Germany, Bethmann-Hollweg reportedly called the 1839 treaty "a scrap of paper" not worth fighting over. Bethmann-Hollweg's efforts were unsuccessful; when German troops entered Belgium on August 4, Britain, as well as Belgium, immediately declared war.

The German advance was halted by the Belgian army at a powerful network of fortresses around Liège. The Germans used 420-mm siege howitzers to reduce the supposedly impregnable fortresses to rubble. Albert, king of Belgium and commander of the army, positioned his forces on a line running north from Namur to Diest, but they could not check the flood of German troops. On August 18, the Belgians retreated north to Antwerp.

The German Second Army under General Karl von Bùlow took Namur the same day. General Alexander Kluck's First Army entered Brussels on August 20 and swung southward to the Sambre River, positioning itself east of Bùlow's forces. The German right wing was now prepared to invade France. With Albert's forces isolated in Antwerp, the Belgian attempt to halt the German advance had failed. However, Belgian resistance had cost the Germans almost a month's time.

It was during this early period of the war that many of the incidents that came to be termed "the rape of Belgium" occurred. While many reports of German brutality were exaggerated for propaganda purposes, some were true. Numerous villages were burned, and hundreds of civilians were murdered in reprisal for isolated cases of resistance by Belgian civilians.