"Patriotism is an instant reaction that fades away when the war starts"

In this quote, Mick Jagger is suggesting that patriotism is often a short lived emotion that rapidly dissipates when faced with the truth of war. It is easy to feel a sense of pride and commitment towards one's nation in times of peace, however when war breaks out and the real horrors of conflict are exposed, that initial sensation of patriotism might disappear. Jagger is highlighting the fact that real patriotism is not simply a temporary feeling, however rather a deep and long-lasting commitment to one's nation, even in the face of misfortune.