Explain what supremacy of EU law is and how it works;

The answer should include consideration of the following:
• Explanation of what supremacy of EU law is and how it works;
• Some consideration of why supremacy was not mentioned in the original Treaties and why was there a need for it;
• Elaboration on the arguments stipulated by the CJEU in Costa and subsequent cases justifying why EU law is supreme.
• Some consideration of how the Member States reacted to supremacy and the need of accepting it.

Any answer should thoroughly explain the judgement in Costa, and provide further arguments in light of other CJEU cases (Van Gend, Factortame, Internationale Handelsgesellschaft, etc).

Decent answers would consider the contrasting views on supremacy between the CJEU and Member States (Germany, Italy or France) as well as explain the UK approach in Factortame. Excellent answers would support the discussion with relevant academic commentary.