"The problem is, of course, the Taliban are a very large organization. It's a syndicate, almost a criminal syndicate in so many ways. Nobody really knows how many there are. Our [intelligence] estimates would put them between 25 and maybe 30,000, max. But they are distributed all over," he said.
"Some of them are in the safe havens where they have an opportunity to rest and refit. Some of them are in transit and many are only in support roles and then some are gun toting, infantry, they are measured in the thousands on any given day."
Allen said he does not buy the notion that Americans do not support the war, and that deep down they know success in Afghanistan is as important as it has ever been. But he said he also knows that 10 long years of war have brought terrible pain to troops, families and friends.
"Whenever I hear of someone who's been killed, my first thought is that there's a family at home asleep that doesn't know yet that their loved one is gone or will never be the same again. That's the first thought I get. And then, of course, writing the letters," he said.
"And when I address those letters to the children, those are the toughest letters to write."
As for what he tells those children back home, he said, "That your father was a hero. That your mother was a hero and that they died in a great cause, and that we'll keep faith with them."