That's when I remembered that I had left the living room window open for some fresh air, and so the cats could also smell and hear the birds they were watching at the feeder. And wouldn't you know, when I went to the window, there was a big black and white neighbor cat sitting on the outer windowsill serenely taunting Ben by his mere presence in Ben's Territory. That, and Ben was probably jealous because the other cat could, if he so chose, actually eat some of the birds at the feeder without having to scratch his way through a wire screen with declawed* front paws. So I got the spray bottle and sprayed the other cat through the screen, and he ran away. But Ben was so riled up that he continued yowling and pacing around the living room with his tail puffed up, and when Ivy walked in to see what was up, he turned on her. The she began her high-pitched scaredy-cat mewling and tried to retreat under the dining table. I was worried we were headed for a bloodbath, so I sprayed them again to distract them, and then tried to herd Ben into the basement so I could close the door. He was so keyed-up that he almost attacked me when I came near him, but I eventually got him in the basement, and after 10 minutes with the door shut and the cats separated, peace was restored, and all three of us were miraculously injury-free. Phew!
*Disclaimer: This is not a procedure that we put our cats through. Each of their former owners did, and since the first cat we got (Ivy) was declawed, we specifically sought out a second cat who had already been declawed so that neither one would have an unfair advantage. I am NOT a proponent of declawing, and would not myself choose to put an animal through such a painful and deforming procedure.