On August 2nd, 1914, under the pretext of forestalling a French attack through Belgian territory, Germany issued an ultimatum to Belgium to allow German troops free movement across its borders. Belgium had 12 hours to respond. On July 24, 1914, the Belgian government had already announced that if war came it would uphold its neutrality.
In reality, German war plans required a quick attack through Belgium into France, scheduled to begin on August 4. While England had no treaty obligation to defend France (though it was generally inclined to do so), England had guaranteed Belgian neutrality per the 1839 Treaty of London. Thus, a German attack on Belgium would mean war with England.
The German plan (it didn’t quite work out this way):
Germany had already began occupation of Luxembourg on August 1st:
Initially, Luxembourg was only a transit point for Albrecht von Württemberg’s Fourth Army. One of the railways from the northern Rhineland into France passed through Troisvierges, in the far north of Luxembourg, and Germany’s first infringement of Luxembourg’s sovereignty and neutrality was the unauthorised use of Troisvierges station. [Luxemburg] protested, but could do nothing to prevent Germany’s incursion.
The next day, Germany launched a full invasion. German soldiers began moving through south-eastern Luxembourg, crossing the Moselle River at Remich and Wasserbillig, and headed towards the capital, Luxembourg City. Tens of thousands of German soldiers had been deployed to Luxembourg in those twenty-four hours. Grand Duchess Marie-Adélaïde ordered that the Grand Duchy’s small army, which numbered under 400, not resist, and, on the afternoon of the 2 August, she and [Prime Minister Paul] Eyschen met the German commander, Oberst Richard Karl von Tessmar, on Luxembourg City’s Adolphe Bridge, the symbol of Luxembourg’s modernisation. They protested mildly, but both the young Grand Duchess and her aging statesman accepted German military rule as inevitable.
In the east, German troops marched into Poland, which did not exist as a country at the time, on August 2nd, and occupied Kalish, Chenstokhov and Bendzin:
With the outbreak of World War 1 in 1914, Poland was quickly invaded by Germany and Austrio Hungary. Many of the significant battles on the Eastern Front during World War 1 were fought in Polish lands, resulting in staggering losses of life among the Poles. With no Polish army to fight for a national cause, approximately two million Poles were conscripted into the Russian, German or Austrian armies, forced to fight one another. With the poor performance of the Austrian Army on the Eastern Front and Emperor Karl’s attempt to exit the war, Germany quickly assumed command of the entire region, seizing Warsaw in 1915 and much of eastern Poland by 1916. While a Council of State was organized for a declared “Kingdom of Poland”, the Germans retained absolute control.