Screening is a defensive security tactic in which a screening force consisting of a picket or outposts is used to hide the nature and strength of a military force, as well as provide early warning of enemy approach, impede and harass the enemy main body with indirect fire, and report on the activity of the enemy main body.
[1] A screening mission seeks to destroy or repel enemy forces when possible, providing early warning in a defensive manner, deny enemy reconnaissance units close-in observation of the main body.
An effective screen can conceal where an army begins and ends, making it hard to flank.
In modern warfare, screening is performed by armoured cars, light tanks and air cavalry.
During the American Civil War, at Gettysburg, Pennsylvania, Maj. Gen. John Buford set the conditions for Maj. Gen. George Meade's success by ensuring the Army of the Potomac occupied the high ground, which destroyed General Robert E. Lee's army when it attacked.