The United States government (as well as the UK and most other European democracies) officially refers to it as “Burma”, a perfectly fine name for the country. Unlike India, whose democratically-elected government deliberately renamed many of its place names in a nationalist effort to assert independence from its British-ruled past, the name “Burma” is simply an anglicized word the locals have always used to refer to the majority ethnic group and the language of the inhabitants. They still refer to their country as “Burma” in verbal conversations.
The name “Myanmar” was arbitrarily hoisted on the country in 1989 by a whim of the repressive military junta that still runs the place. Linguistically, Burmese uses different sounds for the written versus spoken forms of some proper nouns, and “Myanmar” is the sound of the written, formal version of the country name. So why did they change the name we foreigners are supposed to use? Perhaps the ruling junta wanted us to treat their government with more respect.
It’s not an entirely bad name change. Unlike names like “Stalingrad” or “Soviet Union”, which were brand new terms specifically intended to force a new political agenda, there is some logic in asking English speakers to use the pronounced form of the written names. The change is applied consistently to all place names including the (long-time) capitol city Rangoon (Yangon) and the main river, Irrawaddy (Ayeyarwady).
But that is a decision that should be up to the people, not the military. Perhaps someday the citizens of a newly-free Burma will elect representatives who will themselves choose to call their country “Myanmar”, at which point I’m sure the United States and other countries will recognize the change. Until then, I’m going to call it by the name the people used the last time they were free.