Why doesn't military intervention in foreign conflicts work? Or does it?

Because the US military hasn't fought to win a war since WWII. The US Military is the epitome of American myopia. Somehow "we" feel that our way of life and social structures should be placed into the cultures and societies of the world. In the arrogance of our people and politicians, it's been decided that we know all, can do all and others must emulate America's greatness and perfection!!(SARCASM FONT ON) When this country accepts the fact that the military forces are meant to engage in the destruction of those who threaten the welfare and stability of America. Any tool can be used incorrectly, but do not expect a good result from the misuse of said tool.

I think our record has always been spotty, however, in this day and age I believe the biggest impediment is the lack of patience of our elected "leaders" and the media's controversy creating hand wringing which gets the general public all wound up.

It is difficult to compare the type of occupation (rebuilding) today (in both Iraq and Afghanistan) with, for example, the occupation of Japan and Germany after WWII. Having served in Germany, I have to say that by the mid-80s, 40 years later, it was very clear the occupation by allies after the war had been good for Germany.
Both current occupations have been complicated from the start by ongoing armed conflict which destroys many rebuilding efforts. It is doubtful that the current endless seeming occupations, in 40 years or more, would change the nature of the current rebuilding efforts, except to create more problems domestically because of neglecting issues that need funding.

The basic problem is we don't bother to understand what the issues are. We regard the "wars" in the Middle East as new developments. To much of our opposition, they are Act III of WW-I. After WW-I, France and England drew a lot of arbitrary lines in the old Ottoman Empire and defined them as "Nations". These boundaries very deliberately included a religious majority and a minority. They picked a guy they could run from the minority to run the place resulting in internal hatreds (minority government constantly quashing majority population) kept the whole country so busy fighting among themselves they never noticed who was really screwing them. A brilliant post-colonial strategy from which we're now reaping the whirlwind.
Add to that that the fact that the US, after all these years (since 1954 or so), hasn't learned how to fight an asymmetric war. Seventy years on, we are still fighting WW-II (the only even-close-to-success being the first four months after 9/11 - which effort we quickly abandoned in favor of carpet-bombing). We as a nation are flying on instruments here (in the sense that there's no visual reference to the horizon when you've got your head up your ass).

In recent history our intervention has proved less than successful because we have - 1) Tried to achieve strategic goals with "limited war". Limited warfare brings a limited result. 2) Use of military forces in the role of "peacekeepers". Our Armed Forces are not trained or equipped to carry out such roles. These roles are better left to the baby blue berets. There has also been, in my opinion, an up and down level of support from the public and elected officials. Everyone wants the bad guys dealt with, but when the casualties rise, and civilians suffer, it's "oh crap, we've gotta get out of there".

Read Tony Zinni's book "before the first shots are Fired." Know how to get out, before you step in.

My guess would be that we're thinking that we can win hearts and minds BEFORE breaking them.

Because we have men who have never served making decisions and strategy. They are only concerned for their well being...yes...they are political leaders.

This is a timeless question and I have read some really good responses. I agree that we have changed what we do with our military, also the times have changed. After WWII it was the ARMY of OCCUPATION in both Europe and ASIA, then Uncle Joe pulled a fast one that many saw coming and started scooping up territory. Today we "Don't want to look like we are Occupying the country", we don't allow our soldiers to fight to their fullest extent. We make up ROE that are just plain stupid, all in the name of PC. The same people who preach that the "Geneva Convention says you can't shoot at the Mosque" fail to read past the comma, and they set down the rules. Most of them never leave the FOB unless they have a company sized element for a security team. I tend to agree that kill who needs killing, and blow up what needs to be blown up and let the civilians rebuild, or go back to the ARMY OF OCCUPATION!

it depends on how`s in charge. army`s fight. first & for most ! Government changes the rules that govern what mite be done. if the goal is Not cl ear then it is a waste of time. Iraq was a waste of time at the start. there was No clear cut goal to start with, no good plain. some one just throught they would just roll over & play friends still not happing.
.