This was a problem for which we did a lot of drills. The purpose was to prevent an individual or a small group of people from coming aboard to sabotage the ship and its mission. As we practiced it, the intrusion could could be a single individual or a protest with hundreds of people.
When we heard the announcement over the 1MC to “Repel Boarders,” all of us who were on the repelling team were to go to the armory, check out our guns, and receive an assignment. We would all rush to the armory as fast as possible, sign out a rifle or a pistol, plus a specific number of bullets. Then we would race to our assigned locations and watch for anything unusual. The marines, who always had their guns within arm’s reach, would head for their posts immediately.
The ship was particularly concerned about repelling boarders in several situations that I remember. In San Diego, the Chicago’s berth was just a couple of miles from the U.S. Navy Seal Base at Coronado, California. The Seals in training had a continuing challenge, that required them to swim under water to one of the U.S. Navy ships tied up in San Diego Harbor, climb aboard, and plant some broom sticks painted as dynamite on the captain’s table, and then sneak away without being caught. We never had it happen to us, but several of the ships around us did. That, of course, made the captain who received the fake dynamite very angry, and we always tried to be on the lookout for that.
The other place where we were very concerned about the possibility of illegal boarders was Pakistan, which we visited on our 1974 tour of the Indian Ocean. Just before we arrived, Pakistan had lost a war with India and the people were starving. There had been rioting in the streets, and the U.S. Embassy was concerned that this might spill over to the area where we were tied up. So when we were in Karachi, the ship only allowed 1/3 of the crew to have liberty each day, which gave me only two days off to go sightseeing during our six day visit to Pakistan. (Fortunately, we never had any problems while we were there.)