I think the most serious problem today in the world is terrorism. 
                  At the end of 20th centuries , we had a problem of cold-war in our world. There are two rival military groups namely NATO and WASAW. We usually worried about they could make third wrold-war any-time. Now cold war over is over after soviet union was disintegrated since 1989. That is why we no longer worry about another World-war comes again.
                  Today the primary enemy of this world is terrirosm. Terrirosms attacked the innocent people three times as far as i m concerned . There were World Trade Center , New York City in USA , Mumbai in India and Boston in USA recently.
                   Terrorism comes from the clash of civilization  i think . They hate freedom and democratic value . They want to keep living in the old-fashioned world . They never change their-mind-set and their culture. As far as they do not change their point of view and opinion , our world will be faced with very dangerous problems  , i think.