1941 act during World War II
On December 11, 1941, Italy declared war on the United States. The declaration followed the Japanese attack on Pearl Harbor four days earlier, and was made the same day as Germany's declaration of war against the United States. Benito Mussolini publicly gave the announcement in Rome on December 11. Italian Foreign Minister Galeazzo Ciano delivered the news to the head U.S. diplomat in Italy, George Wadsworth II, shortly before Mussolini's statement. 
The Italian, German, and Japanese governments had all signed the Tripartite Pact in 1940, formally allying the three powers with one another. Italy and Germany also signed the Pact of Steel military alliance in 1939. Italy had declared war on the United Kingdom and France on June 10, 1940, as Mussolini implemented the Pact of Steel and tried to use the German alliance to win advantages for Italy. Prior to Hitler's declaration of war against America there was little, if any, doubt that Italy would once again "follow Germany's lead." 
The United States Congress immediately responded by declaring war on Italy and Germany, bringing the United States into the Second World War and involving it in both the European and Pacific theaters of the conflict. 
Background.
On December 7, 1941, 353 aircraft of the Empire of Japan attacked the U.S. naval base at Pearl Harbor and inflicted mass destruction on American life and property, drawing the United States into the Second World War. On December 8, the U.S. declared war on Japan in response to the attack. 
Three days later, at 2:45 PM on December 11, 1941, Italian dictator Benito Mussolini announced—from the balcony overlooking the Piazza Venezia in Rome—that Italy and Germany would "participate from today on the side of heroic Japan" against the United States. That same day, Adolf Hitler made his announcement at the Reichstag in Berlin, stating that while he had tried to avoid direct conflict with the U.S., Germany was obliged to join with Italy to defend Japan under the Tripartite Pact of 1940. Hitler also stated, "After victory has been achieved, Germany, Italy, and Japan will continue in closest co-operation with a view to establishing a new and just order."
A memo that U.S. chargé d'affaires to Italy George Wadsworth II sent to the U.S. Secretary of State on December 9, 1941 confirmed that Italy would follow Germany in declaring war on America, stating: "Opinion in well-informed Rome circles is divided on the crying question of the day whether Germany will declare war on the United States. Italy, it is assumed without question, will follow Germany’s lead whatever it be as a matter of course."
Just before Mussolini's speech, at 2:30 PM, Italian Foreign Minister Galeazzo Ciano called for chargé d'affaires Wadsworth and revealed that Italy was at war with America, to which Wadsworth responded: "It is very tragic." Ciano recorded his thoughts on the public announcement, saying, “It was three o’clock in the afternoon, the people were hungry, and the day was quite cold. These are all elements that do not make for enthusiasm.”
Text of the declaration.
Contrary to Hitler's nearly 90-minute tirade against America, Mussolini's war declaration lasted only four minutes. Still, it was only slightly shorter than President Roosevelt's "Day of Infamy" speech of December 8. Foreign Minister Ciano said the speech was "brief and cutting," and noted its "very pro-Japanese setting." "News of the naval victories" of Japan "has excited the Italian imagination," he remarked.
On December 11, the Axis powers also signed the "No Separate Peace Agreement," pledging each nation to not independently make peace with Britain or America, a decision that theoretically bound Italy, Germany, and Japan to a common fate. Mussolini "expressed no reservations about war with America," and laid responsibility for the conflict at the feet of President Roosevelt. The English-language version of his speech published at the time is as follows:
 ;templatestyles src="Template:Quote_box/styles.css" / ;
This is another day of solemn decision in Italy's history and of memorable events destined to give a new course to the history of continents.
The powers of the steel pact, Fascist Italy and Nationalist Socialist Germany, ever closely linked, participate from today on the side of heroic Japan against the United States of America.
The Tripartite Pact becomes a military alliance which draws around its colors 250,000,000 men determined to do all in order to win.
Neither the Axis nor Japan wanted an extension of the conflict.
One man, one man only, a real tyrannical democrat, through a series of infinite provocations, betraying with a supreme fraud the population of his country, wanted the war and had prepared for it day by day with diabolical obstinacy.
The formidable blows that on the immense Pacific expanse have been already inflicted on American forces show how prepared are the soldiers of the Empire of the Rising Sun.
I say to you, and you will understand, that it is a privilege to fight with them.
Today, the Tripartite Pact, with the plenitude of its forces and its moral and material resources, is a formidable instrument for the war and a certainty for victory.
Tomorrow, the Tripartite Pact will become an instrument of just peace between the peoples.
Italians! Once more arise and be worthy of this historical hour!
We shall win.
References.
 ;templatestyles src="Reflist/styles.css" / ;