yes
No, this is another internet myth. The president has not declared martial law, on 19 March 2012 or on any other day. If there were martial law, you would see U.S. soldiers in the streets arresting large numbers of people and putting them in detention camps. This has not happened, nor is it likely to occur. Further, there is no evidence that Mr. Obama will declare martial law, contrary to partisan claims by some of his political opponents.
Yes
because of the price hike of the gasoline in Libya
The Governor of a state or the Present of the U.S. has the power to declare martial law in case of a state emergency. Either case is debatable in the U.S. Constitution and the state's Constitution.
Hitler threatened to declare war on Czechoslovakia because the Czech government declared martial law.
Maryland - to seize its railroad and protect the Northern capital (Washington D.C)
No
It is not clear what country you are talking about.
Martial law was declared for part of the Civil War by Abraham Lincoln in the state of Maryland. Maryland had many southern sympathizers. It was meant to keep order.
Maryland - to seize its railroad and protect the Northern capital (Washington D.C)
Presidents usually do this during times of war or crisis, but Obama recently stated that he can declare martial law during peacetime. Don't ask why, I'm not sure maybe he just wants power.
"Under what circumstances will you declare martial law?" means the same as "What will it take for you to declare martial law?". "Under what circumstances will black mold form?" means the same as "What conditions will result in black mold forming?" It asks what situation is required for the specified result to take place.