War crimes cannot be deterred by the threat of prosecution against individual perpetrators. War criminals and people who commit crimes against humanity frequently find refuge in countries where legal, political or social conditions offer them safety. In the four decades since the end of the Second World War, preliminary or criminal proceedings have been initiated against thousands of persons believed to have taken part in Nazi crimes, but in the majority of cases the proceedings did not end with convictions either because the suspects were not extradited back to Europe, or there was insufficient evidence against them.
The measures of punishment that may be used against war criminals were set forth in the statutes of the International Military Tribunals (1945-1946 and 1946-1948) as well as in the law of the Control Council of Germany (1945). The Convention on the Non-applicability of Statutory Limitations to War Crimes has been in force since 1970. Despite these instruments, crimes of declared and undeclared wars are known since 1954 but have mainly been unpunished. The trials and sentences of 24 Nazi defendants in Nuremberg in 1945-1946 were provided on ex post facto grounds, no firm legal precedent having been established, and are not universally regarded by international jurists as having certain legal effect. This is not to say that they cannot be invoked, and other instruments have been proposed - such as the International Law Commission's Code of War Offences, prepared in 1954.
War criminals are known to reside in the UK and Canada where local courts do not have the jurisdiction to prosecute war criminals from World War II. They reside in countries in Latin America where frequently they are effectively immune from detection or have political influence.
At the end of W.W.II, the US actively sought out the scientists of Nazi Germany, and from Japan (unit 731, the Ishii unit). Certain generals decided that it was much more important to have the information that these scientists derived from the torturous experiments that each committed against the subjected, conquered populations of the regions they'd occupied, and on some of the prisoners of war, than to try them for war crimes. The US brought hundreds of these scientists to the US, or hid them under the auspices of the allied occupation forces, and allowed them to remain free. Some went on to positions of extreme power and wealth in exchange for the results of their insidious research programs.