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The impact of the post-election reforms has been observed in numerous areas, including ASEAN's approval of Myanmar's bid for the position of ASEAN chair in 2014; the visit by United States Secretary of State Hillary Clinton in December 2011 for the encouragement of further progress—it was the first visit by a Secretary of State in more than fifty years (Clinton met with the Burmese president and former military commander Thein Sein, as well as opposition leader Aung San Suu Kyi); and the participation of Aung San Suu Kyi's National League for Democracy (NLD) party in the 2012 by-elections, facilitated by the government's abolition of the laws that previously barred the NLD. As of July 2013, about 100 political prisoners remain imprisoned, while conflict between the Burmese Army and local insurgent groups continues.

How long was the interval between the visits from the United States Secretary of State officials ?
more than fifty years