When Did We Declare On Pakistan?

From Fox News: Pakistani intelligence officials say a U.S. drone has fired two missiles at a compound in a northwestern tribal region, killing eight militants.
Two officials say the strike happened Saturday in Datta Khel, a town in North Waziristan where the Pakistani military is carrying out a major offensive against militants since June 15.

The officials spoke on condition of anonymity because they are not authorized to talk to media.

It was the second strike this week. On Wednesday, the U.S. drone killed 15 militants there.
North Waziristan is home to local and foreign militants suspected of attacking American and NATO troops across the border in Afghanistan.
U.S. drone strikes are a serious source of tension between Washington and Islamabad. Pakistan denounces the strikes as a violation of the country's sovereignty.  http://www.foxnews.com/world/2014/07/18/us-drone-strike-kills-8-militants-in-pakistan-officials-say/?intcmp=latestnews
Maybe I missed it, but when did we declare war on Pakistan?