Complex sentence: In 1921, the ban was partially lifted after Spain refused to buy Iceland's main export, fish, unless Iceland bought Spanish wines; then lifted further after a national referendum in 1935 came out in favour of legalising spirits.
Split into simpler sentences: In 1921, the ban was partially lifted after Spain refused to buy Iceland's main export, fish, unless Iceland bought Spanish wines. The ban was then modified after a national referendum in 1935 came out in favour of legalising spirits.