World War I
Germany was a land power, and Britain was a sea power. However, Germany was building a large navy, which was seen as a threat to Britain. Germany's generals had decided that the best way to defeat France was to go through Belgium using a plan called the Schlieffen Plan by German Army Chief of Staff Alfred Von Schlieffen. The Germans could then attack the French army at the north side and the south side at the same time. The German Army went into Belgium on August 4. The same day,  the British declared war on Germany. The British had said in 1839 that they would not let Belgium be invaded, and they kept their promise.