As described in a recent radio interview, I began writing the trilogy for my children and grandchildren.
That was my initial inspiration.
But it's much more complicated than that.
I had been retired for around two months, after around 50 years in two vocations. During that time, my wife and I had gone through a lot of catching up: setting up a trust, making financial management decisions and plans, and figuring out where everything was in our home, which itself was a challenge! We still had boxes that had not been opened from other moves; moving stickers were still on some of our furniture and most of our appliances. We found tons of stuff long forgotten, including duplicate tools in great depth caused by years of not being able to find the tool I needed; and in the interest of saving days, would dash to the nearest hardware store to buy another (that I couldn't find later).
My first short story had been published, there was more to come; and I had written a Western entitled The Man Who Loved Blue Star. Not long after I sent it out I received a call from a magazine editor. He had just read my story; said he would publish it, and urged me to use it as an outline for a novel. I was impressed by the thought; and began doing as he and my mentor at the time had suggested. I could create all of the "wisdom" I wanted to pass on to my children and theirs in one book; and the Western "good guy-bad guy" and "good girl-bad girl" brand might work quite well. My children could read it, then pass it on to theirs--when they were around the age of majority, if not before.
After a few months on that path, including many interruptions, I knew what I was doing. It was historical fiction with a number of sub-genres--Blue Star would appear in the centerpiece--and it would be incomplete without a back story and a continuation. I began to do some extensive research, prayed hard, and somehow received the additional inspiration, to significantly expand what had begun as a rather simple task.