1940s-1960s
European colonialism ends with the liberation of 57 countries in Asia and Africa. State sovereignty is an important and sensitive issue for the countries recently freed from the yoke of Western rule and one that will be used for decades to come as an argument against humanitarian intervention.

1971 
India intervenes in a bloody civil war between Pakistan and East Pakistan, which declares independence as Bangladesh. Seven years later Vietnam invades Pol Pot's Cambodia, and in 1979 Tanzania deposes dictator Idi Amin in neighboring Uganda. All three wars are fought under the banner of national interest, but as each also aimed to avert the mass slaughter of civilians, international-law scholars later look to them as precursors -- however faint -- of the humanitarian interventions to come.

1991 
The Soviet Union collapses. The next decade's conflicts don't carry the lofty geopolitical stakes of the Cold War and are more likely to happen within countries' own borders, complicating the prospect of outside forces stepping in.

1994 
Ethnic Hutus begin killing Tutsis in Rwanda. Susan Rice, then an aide on U.S. President Bill Clinton's National Security Council, says of the crisis, "If we use the word 'genocide' and are seen as doing nothing, what will be the effect on the November election?" The United States does nothing, and by July, 800,000 Rwandans are dead.

1995 
Bosnian Serb forces massacre more than 7,000 Muslim men and boys in the town of Srebrenica, while Dutch U.N. peacekeepers look on helplessly.

1996 
Brookings Institution scholar Francis Deng, later the U.N.'s special advisor for the prevention of genocide, co-authors Sovereignty as Responsibility. The influential treatise argues that sovereign states are defined not by the inviolability of their borders -- the assumption of the post-colonial era -- but by their obligation to protect their citizens.

ABDELHAK SENNA/AFP/Getty Images

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