When the Baltic countries (Latvia, Lithuania, and Estonia) became part of the Soviet Union in 1944, was it an occupation or an incorporation?
It was an occupation, in June 15th of 1940 soviet red army killed 3 Latvian border guards and injured 26 people when they illegally entered Republic of Latvia. The "incorporation" happened 2 days after that when the Soviet government issued an ultimatum.
You might want to visit Occupation Museum in Riga, they have many materials on this topic. http://okupacijasmuzejs.lv/en