Presidential Decision Directive 25
Presidential Decision Directive 25 (PDD-25), is an executive order drafted by President of the United States Bill Clinton on May 3, 1994 following a year-long executive agency policy review and bilateral consultations between dozens of members of Congress and the executive branch.
United States involvement in United Nations Peacekeeping Operations
PDD-25 was created to prevent the United States from using peacekeeping operations as the centerpiece of its foreign policy. However, the United States does see the ability of involvement in these operations as a way to advance American and U.N. interests globally. 
PDD-25 created a "vital national interests test" that limited United States involvement in United Nations Peacekeeping Operations, basically leaving involvement in U.N. operations to be handled in a case by case basis. This test limited United States involvement to those operations which had United States military officers in control of United States troops, a mission that was in the best interests of the United States government, and required popular domestic support for the operation.
The order also seeks to reduce United States financial commitment to these operations from 31.7% in 1994 to around 25% in 1996.
PDD-25 was drafted with the knowledge that in the post-Cold War era, new threats to the United States would eventually emerge, and that the United States required a military that would be able to win wars unilaterally.  It represented a continuing negative stance towards a standing United Nations army and prevented the earmarking of specific amounts of United States troops at the disposal of the United Nations international community.
Criticism of the Order
It has been argued that the executive order was drafted by Clinton because of the increasing political pressure and negative press surrounding the failure of the peacekeeping operation in Somalia, where 18 American soldiers in the Unified Task Force (UNITAF) were killed.
PDD 25 was given as the legal basis for deploying American soldiers into Macedonia in 1993-1999, Operation Able Sentry. See www.globalsecurity.org/military/ops/able_sentry.htm
Congress has never been allowed to see PDD 25, which, according to the State Department, is now declassified, as of 2009. The Clinton Library is the repository of this document.
Full text of PDD 25 can be found here on the Clinton Library website.
The Rwandan Genocide
The vital interests tests prevented the United States from actively supporting the peacekeeping operation proposed to end the genocide in Rwanda.
- "PRESIDENTIAL DECISION DIRECTIVE/NSC-25".
- Quynh-Nhu Vuong, J.D. Candidate, School of Law, University of California, Berkeley, 2003, Berkeley Journal of International Law, 21 Berkeley J. Int'l L. 804, p. 812-14
- Saving Strangers: Humanitarian Intervention in International Society, Nicholas J. Wheeler, Oxford University Press, 2000 http://books.google.com.sg/books?id=AYQLyuN4_twC&pg=PA227&lpg=PA227&ots=12_lzrJiSk&sig=9Xl7uwulzXi6AH2rkg_-KTxOarc&hl=en&sa=X&oi=book_result&resnum=6&ct=result#PPA227,M1