With all the stresses of competition for territory, natural resources and political alliances, the major powers had been developing plans for action when not if, war should occur.
The French War Plan was to 1) push back against potential German expansion clearing the west bank of the Rhine River territory, and to 2) show Russia its good intentions as an ally. France planned to put five armies between Switzerland and the Sambre River on the Belgium border. France predicted that Germany would attack through Lorraine and eastern France. Thus the initial finch offensive would be on the right (south) to secure Mulhouse and Colmar in Alsace. They were going to put the first and second armies in Alsace and on the Rhine, and then move into Lorraine. These two armies would advance down the Rhine valley to Koblenz. North of Verdun, the French planned to place three armies to be directed, depending on German actions through the Ardennes or into Luxembourg and Belgium.
The Russian War Plan was to use four armies to clear Prussia, and then move into Berlin. The Russians thought the Germans were less of an immediate threat and after clearing Prussia they would redeploy up to two armies opposite Galicia, and if needed to send two armies to the Silesian front to defend against a German action from the southeast.
British Intentions: Britain had no war intentions as it did not have a formal treaty to support France if it was invaded, but had made provisions to send a small “expeditionary force” to assist the French army, and was prepared to impose a sea blockade on Germany cutting it off from trade and supplies from the outside world.
The German War Plan: was to send the vast majority of its forces against France, with a smaller force to defend itself from an attack by Russia from the east. The strategy is known as the Schlieffen Plan. German would invade France through the neutral Netherlands, Belgium and Luxembourg, curve around to the southeast to trap the French armies – which they assumed would be facing Alsace and Lorraine.
The Schlieffen Plan
On August 4, 1914, Germany invaded Belgium and by August 16th had captured the last of the Liege forts. On August 23-24 the eight German corps attacked the British Expeditionary Force which was supporting the French line on either side of the Belgian town of Mons.
Heavily outnumbered the French and British withdrew to the southeast.
The German Invasion of Belgium.