What Is The Maastricht Treaty?

This is a treaty that was agreed in 1992 by the then 12 member states of the European Union. It came into effect the following year. Under the treaty the EU countries decided to strive for certain common goals subject to the commitment of the countries at national level. Among the aims were political and monetary union. The treaty is more properly known as the Treaty on European Union. Their were wide political implications involving a common defence policy and a central European bank.
Ratification of the treaty was complicated by squabbling over particular clauses. Britain in particular didn't like the implications of the Social Charter. This would have given guaranteed rights to workers in the EU. Eventually the chapter was dropped and other countries signed up to it separately.
Britain also won the right to opt out of a common currency.
Denmark at first turned down the treaty in a referendum but after negotiating the right to opt out of other matters like defence, monetary union and citizenship, decided on a second referendum to go ahead.