The definition of terrorism excludes actions by military forces of a country. Under international law, actions by military forces for the benefit of a state are not considered to be terrorist acts, so if people are acting on behalf of a state or on behalf of the armed forces of a country, then those are not considered to be terrorist acts for the purposes of international conventions or the purposes of this law. Therefore, if the person joins an army of another country, that would not be considered to be terrorism.
On November 19th, 2012. See this statement in context.