Final answer:
Lithuania is the country with the Curonian Lagoon, bordered by the Curonian Spit with the Nemunas River flowing through it. Lithuania is one of the Baltic states that gained independence from the Soviet Union in 1991 and joined the EU and NATO in 2004.
Step-by-step explanation:
The country that has the Curonian Lagoon, separated from the Baltic Sea by the Curonian Spit, with the Nemunas River flowing through it, is Lithuania. The Curonian Spit is a 98 km long, thin, curved sand-dune spit that separates the Curonian Lagoon from the Baltic Sea coast. The Nemunas River is the largest river in Lithuania and one of the biggest in the Baltic states.
The Baltic states, including Lithuania, gained independence from the Soviet Union in 1991 and have been working towards economic integration with Western Europe. Lithuania, together with Latvia and Estonia, joined NATO and the EU in 2004, attesting to their transition to market economies and democratic governments.