2 Samuel 11:1
And it came to pass, after the year was expired, at the time when kings go forth to battle, that David sent Joab, and his servants with him, and all Israel; and they destroyed the children of Ammon, and besieged Rabbah. But David tarried still at Jerusalem.
At the time of the war against Ammon, David stayed in Jerusalem while Joab and his servants with him and all Israel went out to destroy the children of Ammon and besiege Rabbah. The outcome of the war appeared certain, and David was at ease and unguarded. In self-security he let go his hold on God and yielded to temptation, bringing guilt upon himself.
His fall came through indulgence of appetite and passion, by which Satan leads even a person especially favored by God into God's displeasure. He turned from God's teachings and forsook God's Word, thereby staining a previously fair reputation. While he upheld the Word he stood firm, but forsaking it brought consequences.
The account this verse introduces continues with David ordering Joab to place Uriah in the fiercest fighting and to withdraw support from him. Joab obeyed, and Uriah was killed by the Ammonites' sword. David arranged Uriah's death and accepted the report of it, showing how far he had fallen into sin.
In this David used his God-given authority contrary to God's law by commanding Joab to act. Joab sinned by obeying the king rather than God. As God's chosen leader, who should have upheld God's law and been a terror to evildoers, David's own violation of its precepts was especially harmful. His act strengthened the hands of those who did evil.
His fall exemplifies a man favored by God and entrusted with great responsibility being overcome by temptation. Its record warns readers that even believers may fall. Yet through God's grace they can rise again and renew the struggle against sin.