Miyamoto Mushashi was a Japanese swordsman, strategist and author that died in 1645. His contribution of the Book of Five Rings is his most notable writing for defensive mindset. His 61 documented duels are the basis of his expertise.
In many ways, the wisdom of Musashi can be found throughout history in every warring culture. None of the concept are new, but they have also not been discarded, mostly because they are nearly universal and timeless.
His book relates to the sword and direct combat with the sword, but quotes like "it will seem difficult at first, but everything is difficult at first" is a truth that expands beyond even defensive mindset. "See distanced things as if they were close, and take a distanced view on close things". Take a s backand evaluate what is going on. I think we can all benefit from this view.
Some of his sword skills can ve translated directly to the firearm: "Grip the long sword with a rather floating feeling in your thumb and forefinger, with the middle finger neither tight nor slack, and with the last two fingers tight. It is bad to have play in your hands." This is the essence of controlling the end of any lever and can be applied to the firearm as well as the sword. "If you try to wield the long sword quickly you will mistake the Way. To wield the long sword well you must wield it calmly." This makes sense with a firearm as well, given that mistakes generally happen because we rush the process of shooting.
Some of the mimic the military mantra of speed, surprise and violence of action. "In the Way of dueling, however, you must always be intent upon taking the lead and attacking" speaks to seizing the initiative and making your opponent react to you. "Because you can win quickly by taking the lead, it is one of the most important things in strategy." These are still point taught to all leaders in the military. "We must defeat him at the start of his attack, in the spirit of treading him down with the feet, so that he cannot rise again to the attack." Act decisively and leave no room for your opponent to regroup.
Finally, he was a great proponent of training and practice. "You must learn this through repetitive practice." "You must research sufficiently to realize this." Keep training and get better. Read the book and see some timeless wisdom.