Final answer:
Estonia is the country that borders the Baltic Sea between Russia and Latvia. It gained independence from the Soviet Union in 1991 and joined NATO and the EU in 2004.
Step-by-step explanation:
The country that borders the Baltic Sea, between Russia to the east, and Latvia to the south, is Estonia. After declaring independence from the Soviet Union in 1991, Estonia, along with the other Baltic states of Latvia and Lithuania, transitioned away from Soviet connections and worked towards integrating with Western Europe. All three Baltic states were welcomed into NATO and the EU in 2004, shifting from planned economies under Soviet rule to market economies with democratic governments.