Lead elements of German forces crossed over into Belgium, hoping to capture as much as they could without resistance. The invasion of Belgium represented an essential piece of the German Schlieffen plan, whereby large elements of their army would bypass the fortified Franco-German border through Belgium and Luxemburg in an effort to outflank the French army (you can read more about the Schlieffen plan in the blog post of August 2).
An integral element of the invasion was a special strike force of about 30,000 men who had the sole mission to attack and neutralize the forts at Liège, a fortified Belgian city which stood on the Meuse river and commanded the main avenue through which the right wing the German army (their key to victory as pegged by the Schlieffen plan) had to pass.
Lead elements of this force came upon the forts at Liège on August 4. Though they attempted to capture them, they were repulsed by defenders . . . the Belgian troops had already fully manned the defenses. Nevertheless, German General Erich Ludendorff had sent a cavalry force to the north of the forts, and its presence there worried the Belgian leader, General Gérard Leman, that they might soon be surrounded. Leman therefore sent away his mobile defense force to join the main Belgian army. In doing so, he lost any chance he had of keeping the German siege guns at a safe distance. Lemay might be able to delay the Germans, but in a siege it would only be a matter of time.
Simultaneous with the invasion forces, Britain had sent to Germany asking for assurances of respecting Belgian neutrality. Receiving an unsatisfactory answer, Britain declared that a state of war existed between Britain and Germany from 11PM of August 4.