Is the Sun Tzu theory still useful for modern warfare?

Yes, 100% and has been shown to stand the test of time (though some may need help with the original wording). The ideals have spread to other areas beside warfare such as business, sports, and politics and is an excellent work literature...as long as you don't expect it to solve all of life's problems

what has this to do with dogs child

Yes. Frankly I think the first paragraph at this link explains how very effective using "soft power" to win, VERSUS going to actual war still works. Look at the pressure Trump & other countries (united) are putting on IRAN vs, going into a direct conflict. (Their economy is crumbing.)

https://jamesclear.com/sun-tzu-habits

Well, it's not a "theory" so much as it's an entire book full of them. But yes, it's still useful.

From what I've heard from a few graduates, it's recommended reading (or at least it was when they attended) at the Citadel, West Point, and the Air Force Academy. Probably others as well.

When the political scientist Ivan Arreguín-Toft did the calculation a few years ago, what he came up with was 71.5 percent. Just under a third of the time, the weaker country wins. Arreguín-Toft then asked the question slightly differently. What happens in wars between the strong and the weak when the weak side does as David did and refuses to fight the way the bigger side wants to fight, using unconventional or guerrilla tactics? The answer: in those cases, the weaker party’s winning percentage climbs from 28.5 percent to 63.6 percent.