What were some of the positive and negative aspects for the united states of joining post- world war ii international organizations?

Positive:

The years following the end of WW2 saw the United States consolidating as a superpower and the emergence of a global actor only equaled by the Soviet Union at the time. Efforts to prevent further conflicts in global scale resulted in the creation of the United Nations. The headquarters of many other multilateral organizations was lead by the US foreign policy that formerly didn't play such an active role

The "Marshal Plan" became a key for reconstruction of the European economies and it saw an emergence of financial institutions that fostered cooperation and development to many countries. This includes the World Bank, International Monetary Fund. This multilateral organisms were key to reconstruction of countries damaged by war.

Negative:

The policy of containment, and later the Cold war generated local conflicts, where famous cases like Vietnam generated a bad image to the World where Us should have not been involved

to prevent the spread of communism, saw the formation of the NATO, that nowadays is seen as an intromission from the US into European politics especially in the sphere of military and defense. Nato generates a huge military  budget and relies on American troops when Europeans could be more involved in solving their particular issues and being more autonomous

he didn't think it was ethically wrong. he thought it was not going to work and it was not going to be of any assistance. he wrote about that in his memoirs.