The modern version of peacekeeping really began in 1956 when that UNEF force was first interposed between the two sides after the Suez War, after the shooting stopped, and in a sense it represented a SovietAmerican veto over the use of force by the other powers.
Bloomfield, Lincoln P.
"Conflict Resolution: U.N. Nonfighting Forces,"
Naval War College Review: Vol. 21
, Article 4.
Available at: http://digital-commons.usnwc.edu/nwc-review/vol21/iss4/4