This has been lifted from comments and made into a post. It is by Altandmain.
The US hasn’t fought a serious opponent since WW2. Even then, the US vastly overstates it role and understates the USSR’s role in defeating Germany.
Likewise, the UK had this problem. The UK was not prepared for WW1. It also suffered from that problem in WW2. The reason is because it was focused on imperialist colonial wars. It’s military in early parts of WW1 and WW2 didn’t do so well at first and had to undergo a very steep learning curve.
The US has this problem now as well.