Complex sentence: Such proposals have been made since the 1990s and since 2013 an agreement between the United States and the European Union has been under negotiation: the Transatlantic Trade and Investment Partnership.
Split into simpler sentences: Such proposals have been made since the 1990s. Since 2013 an agreement between the United States and the European Union (EU) has been under negotiation: the Transatlantic Trade and Investment Partnership.