What Makes Unemployment Low or High?

I believe immigration is a huge factor to why unemployment is so high. The UK joined the EU in September 2007. All of a sudden the flood gates were open and the Polish, Romanian and every other EU country were allowed in. The goverment realised they had screwed up so now its alot harder to get in but the damage has already been done. We now live in a society which is forever growing but not enough jobs to go around. I don't blame the people only the goverment for letting it happen in the first place.