Final answer:
The recruitment poster illustrates the German invasion of Belgium during World War I, which was a significant event leading to the UK's declaration of war against Germany.
Step-by-step explanation:
The recruitment poster in question refers to the German invasion of Belgium during World War I. This invasion occurred in August 1914, after Belgium refused to allow German troops to pass through its borders en route to France. The German invasion of Belgium was a key event that prompted the United Kingdom to declare war on Germany, as Belgium was a neutral country and the violation of its neutrality was seen as an outright act of aggression. Germany believed that Belgium would grant them easy passage and was taken aback by the fierce resistance they encountered.
The Schlieffen Plan, which was predicated on a rapid movement through Belgium to outflank French defenses, faltered as a result of Belgian and later British and French resistance. Consequently, this action galvanized public opinion against Germany and contributed to the shaping of early World War I dynamics.