The second Iraq War had been raging for almost six years when I went to cover the March on the Pentagon in 2007. The march started in Washington DC where protesters were met by counter protesters, mostly veterans of the Vietnam War, then proceeded to the Virginia State Line where the protesters were met by the Virginia State Police who denied them entry to the bridge into the state— where the Pentagon actually is. There was one person taken into custody, apparently for incitement.
When the uneventful march back reached DC proper, a few— it’s always just a few— broke off and started knocking over traffic barriers and blocking traffic. They were met by DC Metro Police and they scattered. There was no violence or property damage, and I only witnessed one arrest. Law enforcement in the DC area is complicated, and this short march covered at least four jurisdictions.