They point the cannon at you

Here Fogerty changes the setting, tone, and mood of the entire song, simply by switching into the second person: now those cannons are pointed at you.

Lyrically, John Fogerty is doing something pretty interesting here. In the previous line, he mentioned "Hail to the Chief," which is played as a sign of respect (or a salute) to the President of the United States. This line brings to mind the 21-gun cannon salute, which is often used at official state functions as yet another sign of respect for the president. 

The cannons begin firing during the four ruffles and flourishes that precede "Hail to the Chief." The 21-gun salute is also fired at military funerals. Though cannons, an instrument of warfare, are used in the ritual, it is by no means meant to be antagonistic. Rather, the cannon salute is fired as a sign of respect. 

But in this line of the song, Fogerty adroitly changes the tone and the setting. Rather than being fired in salute, the cannon is being pointed "at you." The war is being brought home to the public in this line, bringing to mind the Armed Forces draft in place during the Vietnam War. The speaker in the song is no longer at an official state function; he is in the trenches, at war.