Complex sentence: Before the collapse of the soviet union, Iran was the only country which officially observed ceremonies of Nowruz but as soon as the Central Asian and Caucasus countries gain independence from Soviets, they also announced Nowruz as of their national holiday.
Split into simpler sentences: Before the collapse of the Soviet Union, Iran was the only country that officially observed the ceremonies of Nowruz. When the Central Asian and Caucasus countries gained independence from the Soviets, they also declared Nowruz as a national holiday.