Defacing the Humble Stone

 is an unmarked quest in Fallout: New Vegas.

Quick walkthrough
When visiting the War Memorial at Boulder City, you will meet an NCR soldier, private Kowalski. He is paying his respects to his brother, who died during the Battle of Hoover Dam.

Aim at the Memorial and Kowalski will become angry, shooting it will make him furious. Keep being disrespectful or apoligize, your choice.