The gathering took place in part to protest the twelfth anniversary of the war in Afghanistan. A similar protest and set of arrests occurred last year at the memorial. In the video embedded below, a veteran tells police that they are there to bring awareness to the "immense atrocities" committed in Vietnam, and that "some of you are so much younger, and history has been denied you, so you just need to know why we are here."
And this had abso-fricken-lutely nothing to do with the closures of national parks and monuments during the government shutdown. This happened last year and will probably happen again next year as this group of Vietnam vets engage in civil disobedience to get their message out.