Task: Question Answering
Input (English): Passage: Britain's declaration of war against Nazi Germany in September 1939 included the Crown colonies and India but did not automatically commit the Dominions of Australia, Canada, New Zealand, Newfoundland and South Africa. All soon declared war on Germany, but Ireland chose to remain legally neutral throughout the war.After the Fall of France in June 1940, Britain and the empire stood alone against Germany, until the German invasion of Greece on 7 April 1941. British Prime Minister Winston Churchill successfully lobbied President Franklin D. Roosevelt for military aid from the United States, but Roosevelt was not yet ready to ask Congress to commit the country to war. In August 1941, Churchill and Roosevelt met and signed the Atlantic Charter, which included the statement that "the rights of all peoples to choose the form of government under which they live" should be respected. This wording was ambiguous as to whether it referred to European countries invaded by Germany and Italy, or the peoples colonised by European nations, and would later be interpreted differently by the British, Americans, and nationalist movements.In December 1941, Japan launched, in quick succession, attacks on British Malaya, the United States naval base at Pearl Harbor, and Hong Kong. Churchill's reaction to the entry of the United States into the war was that Britain was now assured of victory and the future of the empire was safe, but the manner in which British forces were rapidly defeated in the Far East irreversibly harmed Britain's standing and prestige as an imperial power. Most damaging of all was the Fall of Singapore, which had previously been hailed as an impregnable fortress and the eastern equivalent of Gibraltar. The realisation that Britain could not defend its entire empire pushed Australia and New Zealand, which now appeared threatened by Japanese forces, into closer ties with the United States. This resulted in the 1951 ANZUS Pact between Australia, New Zealand and the United States of America. 
Question: What location was the city that was seen as impregnable fortress considered to be the eastern equivalent of?
Output (English): Gibraltar.