Arise, go forth. The occasion called for action, and Joab bluntly and fearlessly told David exactly what he ought to do.
I swear by the Lord. In uttering this solemn oath Joab was not making a threat that he would lead the people in revolt against David, he was only calling attention to an unpalatable truth. The situation was fraught with danger. A large part of the nation had already turned from David and had supported Absalom in his efforts to take David’s life and his throne. And now David was on the verge of alienating those who had continued loyal to him and making them also his enemies.
That befell thee. Joab predicted that David, by his unnatural conduct, was precipitating for himself the greatest crisis of his life. He used strong words, but they were necessary in order to rouse the king from the selfishness and foolishness of his grief.