has the recent actions of the Unites States made the enforcement power of the UN obsolete? Has it become another League of Nations that cannot enforce any decisions?

To a large degree yes.

The UN will never suceed unless all parties work toward making it suceed as long as one party is determined to ignore anything not in its favour then it is screwed.

has the recent actions of the Unites States made the enforcement power of the UN obsolete? Has it become another League of Nations that cannot enforce any decisions?

Two words: Certainly not.

The recent actions of the U.S. was done under the name of America's National Security and requires no international consesus for action.

But stating that the UN as obsolete is unacknowledging and ignoring the many nations who contributed greatly to the development of impoverished countries around the world.

The UN still has its place in the world. But it does have to ring some serious changes.
The big question is peacekeeping. What will become of the UN's peacekeeping role?
We all know it's no secret that peacekeeping missions are expensive, inefficient and on many occasions had led to disaster. There has to be a better way to do this.
The UN is particularily useful in coordinating NGO relief efforts in crisis zones. You need an authority to get many of these groups, whose missions are exactly the same, to do the work without conflict of jurisdiction etc.

I agree that the UN still has a place.

I mean, eventually we will go to war with a country that everybody agrees should be taken care of.

Only partially obsolete.

Organiztions such as WHO and the Red Cross are still needed, but it's military support is lacking because of recent actions. It's security council includes countries that are suspected of harboring terrorists.

It's never going to be as important as the creators in the 40s and the wide-eyed liberals in the 70s wished it would be.

Now the UN is less important than the Red Cross in international affairs.

Closer to the point when was the UN effective

the UN has been effective in helping 3rd world countries. however, is it an effective mediator of international conflicts?

Effective helping 3rd world countries Cmon you must be kidding. Most help to these countries are from aid agencies The Un failed in Korea somalia Zimbabwe Yugoslavia Rawanda, did nothing when suddam slaughtered thousands of Kurds, just to name a few. The Un in my opinion is a big bluff if the "BADDIES" call their bluff they are almost powerless. Never have they said if you do not killing people we will take action and carried it out effectively they usually back down. Now I am not saying that they are in an easy situation they cannot really enter a country without government request otherwise it would be invading another country which is something the UN like to avoid, far enough. But remember in most of these situations the in power government is the source of the corruption and or problem. I think the UN need to grow some balls so to speak or pack up. Eg the UN did nothing in East Timor the Australians and New Zealanders "went in" then after all was said and done recieved UN backing. The 1st and 2nd Gulf wars. US led UN backed. To me the UN is little more than a weak very expensive baby sitter! They still believe right or wrong I cant say that the pen is mightyer than the sword,but 1 thing is for sure their bark is defently worse than their bite!!!
Have a good weekend