After the Japanese attacked Pearl Harbor, the Congress of the United States formally declared war on Japan. After Germany declared war on the United States, the Congress then formally declared war on Germany. However for all subsequent conflicts that the United States has been involved (Korea, Vietnam, Grenada, Somalia, Iraq, Afghanistan), there has been no formal declaration of war. Instead, these conflicts have been justified as UN police actions, UN peacekeeping activities, requests by local governments, security threats, etc. Congress has passed resolutions but no declarations. It now appears that the position of the President as Commander-in-Chief of the armed forces has usurped the power of Congress to control whether the US goes to war or not.
What you may be confusing in this case is what is termed the War Powers Act which gives the President latitude to actually engage in military operations for up to 90 days before declaring war or going to Congress. Part of this stems from the face that Vietnam avoided this with the Gulf of Tonkin resolution which then President Lyndon Johnson used to initially get Congress to accede into allowing soldiers to be sent to Vietnam. Later there were disputes about the accuracy of the attacks cited to get the resolution passed, note the end of the article which mentions the major points. While the War Powers Act has been side-stepped since its passing, the discussions on this came up most recently after the September 11th attacks, a good read on this at Slate which goes over the War Powers act and some of the discussions around sending troops into combat.
So to reiterate my comment, its not like it used to be but times and the methods of war have changed since the Constitution and the Presidents powers have been written.