Sentence:  So confident was he that he ignored the warning of his two British advisers to post <WSD>pickets</WSD> to watch the river, and even withdrew those they had placed there. 
Meaning of the word marked with <WSD> tags:  One of the soldiers or troops placed on a line forward of a position to warn against an enemy advance; or any unit (for example, an aircraft or ship) performing a similar function.