why USA doesn't invade North Korea?

Quite simply, because we would likely lose. The US has not be able to hold any substantial territory in Southeast Asia (or the middle east for that matter), for any extended period after winning the major battles. Afghanistan, Vietnam, Cambodia, Lebanon, Irag are all recent examples of winning the battles and losing the war. Given the size and motivation of the North Korean military, the long term loss would most probably come with 10's if not 100's of thousands of US casualties. Not a good strategy.

Hi so just how stupid are you i wonder. why. yes why start a third world war about nothing that has anything to do with the rest of the world. they have lived within there own borders for the last 50 + years. just leave them alone back off and let the sleeping dog lie.

Two words really - why bother!! More words - invading another sovereign country shouldn't ever be an option. If a country invades, they are usually automatically in the wrong - have committed a criminal act and nobody appointed America as the one country there to police the world. Trump/America needs to look to their own country and sort that out before it embarks on an invasion of another country.

Because North Korea has nuclear weapons and the chances of the US hitting them all during a first strike are slim.

Once a first strike ceased and the North saw they had some bombs still intact they would immediately be launched at South Korea and Japan. Those Premiers know this and it is their influence on the POTUS that keeps the situation from escalating from that side.

One word: China.

it has done before and threatened to nuke China then McArthur tried to convince the American government to nuke China I repeat, nuke so N Korea are well aware of the capabilities of the USA and the USA are in south Korea marching up and down the border sporting their military gear so lets get things straight the situation is caused by the USA invading N Korea would be a catastrophic mistake so they parade a few bombs and let loose a couple of unarmed missiles the USA has a huge amount they parade in front of Russia and in the China sea and anyone else like Iran in the Gulf or a base in nearly every country in the world that's OK they are not there selling flowers the have guns do you get the picture you are the threat buddy and the world is waking up

China has already informed Trump that if the US launches the first strike China will defend N Korea. One of the last things China wants or needs is S Korea with its US military bases on its land border.

Because China. They may hate North Korea at the moment but they need a buffer so US bases aren't on their doorstep. Bush ain't in office anymore we actually need them to try something first and have a reason. Also nukes they're never good to have flying at you.

Because China, for some reason, is willing to defend them if the US does invade.

Because China will get very upset. So no more cheap electronics.