In the wake of the September 11, 2001, terrorist attacks, there was a great deal of concern expressed about terrorist "sleeper cells" in the United States. Sleeper cells can be individual terrorists or groups of terrorists who blend in with society at large; they remain inactive, even for years, until they receive orders to carry out their mission. Some of the perpetrators of the september 11 attacks belonged to such sleeper cells.
Widespread concern over terrorist sleeper cells fueled suspicion that some U.S. citizens were knowingly providing aid and comfort to terrorist cells located in the United States. Aid and comfort was allegedly provided by shielding the identities of terrorists from U.S. authorities, and providing funds, transportation, and other forms of assistance to terrorists who plotted against U. S. interests.
In the subsequent U.S. military action against the Taliban government in Afghanistan and members of the al Qaeda terrorist organization located there, which started in October 2001,U.S. forces captured John Walker Lindh, a 20-year-old American citizen who was trained by and was fighting for the Taliban against the U.S. government. The Walker Lindh case garnered enormous coverage in the press, with many claiming that Walker Lindh's role as a combatant for the Taliban was tantamount to treason as it gave aid and comfort to enemies of the United States." —http://legal-dictionary.thefreedictionary.com/—