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Britain declared war on Germany because Germany attacked Belgium in order to strategically invade France. Britain vowed to protect Belgium, so Britain joined World War I.
Significant for Great Britain, the war was not global yet as the US was not in it; and Germany would not invade Britain since they did not control the air.
Austria, Czechoslovakia, and Poland (The U.K. declared war 3 days later, after Hitler ignored a British ultimatum to leave).
France. Germany and France share a border.
Poland. Germany had previously agreed in the Munich Agreement that if they were allowed to annex Sudetenland of Czechoslovakia they would not invade anywhere else. The Agreement was signed by Germany, Italy, Britain and France. Italy was allied with Germany so Britain and France declared war when Hitler invaded Poland in direct violation of the agreement.