Who was the primister when war was declared?

To provide an accurate answer, I would need to know which specific war you are referring to, as many conflicts have been declared throughout history. For example, in World War I, the British Prime Minister when war was declared in 1914 was Herbert Asquith. In World War II, Neville Chamberlain was Prime Minister when Britain declared war on Germany in 1939. Please specify the war for a more precise answer.