Complex sentence: Immigration to Denmark increased rapidly during the 1960's as the manufacturing economy expanded and the demand for labor increased, however at the end of the decade immigration policy became more stringent, greatly reducing the number of immigrants arriving in Denmark.
Split into simpler sentences: Immigration to Denmark increased rapidly during the 1960's as the manufacturing economy expanded and the demand for labor increased. At the end of the 1960's immigration policy became more stringent, greatly reducing the number of immigrants arriving in Denmark.