EVERYONE is a threat. Seriously.
The military does not evaluate other nations based upon what we think they might do, they evaluate them based upon what they COULD do to the USA if they decided to do it.
in the 1920s the Great Britain and the British Empire was considered the biggest threat. All those nations, all those people and resources, the massive British Navy and Canada just to the North of the USA all added together made them the biggest threat. So we had a plan to deal with that should it come true. (Starting by invading Canada.) The second biggest threat was the rapidly growing Japanese Military. We had a plan to fight them too. We never needed the plan to fight the British but we did use the plan we had in place to fight the Japanese.
Today we look around and what nations could cause us the most harm? Russia and China. We have a plan to deal with just Russia, a plan to deal with just China and a plan to deal with both of them at the same time. There are also variations of the plan such as a low level war once again like Korea, Vietnam, Iraq or Afghanistan or a naval war where all the fighting takes place at sea. Worst case is a full scale nuclear war.
As far as your last comment, the USA has been cutting back on its military capabilities since the Gulf War. We have fewer ships than we had in 1917, we are retiring ten and only building five a year so the Navy is continuing to get smaller. The average age of the air fleet is 25 years old with some planes 50 years old. We have less than 100 B52 bombers, around 60 B1 bombers and 19 B2 bombers. We are retiring a lot of fighter jets. We got less than 200 F22 fighters to replace over 1000 F15s. We are planning on getting somewhere around 2400 F35s to replace all kinds of different planes in three branches of service. Not enough for a one for one replacement. AND we are retiring planes before the replacements have arrived. We have NO/NONE/NADA fighters in development so the F22 and F35 are going to be the last new fighters for decades. We have cut back on our nuclear forces under SALT and START treaties with the Russians. That would be great if all we had to worry about is Russia but what about China? They could have thousands of warheads stored in their tunnels that we do not know about. Lastly, we have over invested in aircraft carriers. They only have an effective combat range of 600 miles so can be taken out by anti-ship missiles with ranges greater than that. What we need are subs. Nimitz himself said after WW TWO the most important weapon in the war was the submarines (Not the carriers.)
What I think you are going to see is China moving on the islands under dispute with Japan, Philippines and Vietnam. And it is going to try to take its rogue state of Taiwan back under direct control. The USA would bet nuts to get into a war over it because it can not fight a close in war with China. the USA will have to try diplomatic tactics and that will let the world know that China is #1...