The Congressional Research Service has issued a report entitled (45 page pdf) Instances of Use of United States Armed Forces Abroad, 1798-2007, updated January 14, 2008 [1] .  It reviews hundreds of instances in which the United States has sent military forces abroad in situations of military conflict or potential conflict to protect U.S. citizens or promote U.S. interests. The listed deployments vary in size and length, legal authorization and significance. In eleven separate cases listed in bold-face type the U.S. formally declared war against foreign nations; but for most the status of the action under domestic or international law hasn't been addressed.  A sample entry:

"1798-1800      Undeclared  Naval  War  with  France.    This  contest  included  land actions, such as that in the Dominican Republic, city of Puerto Plata, where marines captured a French privateer under the guns of the forts. Congress authorized military action through a series of statutes.
1801-05          Tripoli.   The   First   Barbary   War   included   the U.S.S. George Washington and Philadelphia affairs and the Eaton expedition, during which a few marines landed with United States Agent William Eaton to raise a force against Tripoli in an effort to free the crew of the Philadelphia.    Tripoli  declared  war  but  not  the  United  States, although Congress authorized U.S. military action by statute."