When did the US enter the war?

In your question, you do not specify which war you are asking about.  However, this question is usually asked with regard either to WWI or WWII.  I will answer for each of these two wars.

The United States officially entered WWI on April 6, 1917.  At this point, the war had already been going for years, having started in late July of 1914.  The US had tried to avoid becoming involved in this war and had succeeded for years.  However, President Wilson eventually felt as if he had to bring the country into the conflict.  He did this largely because Germany decided to resume its unlimited submarine warfare even though the US had warned it not to do so.  Wilson also asked for war because of the Zimmermann Telegram, in which Germany had tried to get Mexico to enter the war on its side.  Germany had proposed that , in return for its help, Mexico would get back the land it had lost to the US in the Mexican-American War.  For these reasons, the US entered the war on April 6, 1917 when Congress gave final approval to a declaration of war on Germany.

The United States entered WWII on December 8, 1941.  In this war, too, the US was a latecomer as the war had been going in Europe since September of 1939.  The US entered the war as the result of the Japanese attack on Pearl Harbor in Hawaii.  The Japanese attacked on December 7, 1941.  The next day, President Franklin D. Roosevelt asked Congress to declare war on Japan and Congress complied.  On December 11, 1941, the day after Germany declared war on the US, the US reciprocated and declared war on Germany as well.

So, for the two major wars of the 20th century, the US entered WWI on April 6, 1917 and WWII on December 8, 1941.