<P> Two Western powers, the United Kingdom and France, gave guarantees to Poland that they would declare war if Polish independence came under threat, as presented in a statement to the House of Commons by British Prime Minister Neville Chamberlain 31 March 1939 (formalized by the British 6 April 1939; not ratified until 4 September 1939 by the French): </P> <P>... in the event of any action which clearly threatened Polish independence, and which the Polish Government accordingly considered it vital to resist with their national forces, His Majesty's Government would feel themselves bound at once to lend the Polish Government all support in their power . They have given the Polish Government an assurance to this effect . </P> <P> I may add that the French Government have authorised me to make it plain that they stand in the same position in this matter as do His Majesty's Government . </P> <P> Although they honoured these guarantees by declaring war soon after Germany's Invasion of Poland on 1 September 1939, and although the dominions of the British Empire quickly followed suit, so little practical assistance was given to Poland, which was soon defeated, that in its early stages the war declared by Britain and France was described as a "Phoney War". </P>

Why did britain and france declare war on germany in september 1939