The most recent enlargement of the European Union saw Croatia become the European Union's 28th member state on 1 July 2013. The country applied for EU membership in 2003, and the European Commission recommended making it an official candidate in early 2004. Candidate country status was granted to Croatia by the European Council in mid-2004. The entry negotiations, while originally set for March 2005, began in October that year together with the screening process.

The accession process of Croatia was complicated by the insistence of Slovenia, an EU member state, that the two countries' border issues be dealt with prior to Croatia's accession to the EU. Croatian public opinion was generally supportive of the EU accession process, despite occasional spikes in euroscepticism.

Croatia finished accession negotiations on 30 June 2011, and on 9 December 2011, signed the Treaty of Accession. A referendum on EU accession was held in Croatia on 22 January 2012, with 66% of participants voting in favour of joining the Union. The ratification process was concluded on 21 June 2013, and entry into force and accession of Croatia to the EU took place on 1 July 2013.The EU was thus at its largest territorial extent from 2013 until 2020, when the United Kingdom left the union.

Croatia ( (listen), kroh-AY-shə; Croatian: Hrvatska, pronounced [xř̩ʋaːtskaː]), officially the Republic of Croatia (Croatian: Republika Hrvatska, (listen)), is a country at the crossroads of Central and Southeast Europe. It shares a coastline along the Adriatic Sea, and borders Slovenia to the northwest, Hungary to the northeast, Serbia to the east, Bosnia and Herzegovina and Montenegro to the southeast, and shares a maritime border with Italy to the west and southwest. Croatia's capital and largest city, Zagreb, forms one of the country's primary subdivisions, with twenty counties. The country spans an area of 56,594 square kilometres (21,851 square miles), with a population of nearly 3.9 million.

The Croats arrived in the 6th century and organised the territory into two duchies by the 9th century. Croatia was first internationally recognised as independent on 7 June 879 during the reign of Duke Branimir. Tomislav became the first king by 925, elevating Croatia to the status of a kingdom. During the succession crisis after the Trpimirović dynasty ended, Croatia entered a personal union with Hungary in 1102. In 1527, faced with Ottoman conquest, the Croatian Parliament elected Ferdinand I of Austria to the Croatian throne. In October 1918, the State of Slovenes, Croats and Serbs, independent from Austria-Hungary, was proclaimed in Zagreb, and in December 1918, merged into the Kingdom of Yugoslavia. Following the Axis invasion of Yugoslavia in April 1941, most of Croatia was incorporated into a Nazi installed puppet state, the Independent State of Croatia, which committed genocide against Serbs, Jews, and Roma. A resistance movement led to the creation of the Socialist Republic of Croatia, which after the war became a founding member and constituent of the Socialist Federal Republic of Yugoslavia. On 25 June 1991, Croatia declared independence, and the War of Independence was fought for four years following the declaration.

A sovereign state, Croatia is a republic governed under a parliamentary system. It is a member of the European Union, the United Nations, the Council of Europe, NATO, the World Trade Organization, and a founding member of the Union for the Mediterranean. An active participant in United Nations peacekeeping, Croatia has contributed troops to the International Security Assistance Force and took a nonpermanent seat on the United Nations Security Council for the 2008–2009 term. Since 2000, the Croatian government has invested in infrastructure, especially transport routes and facilities along the Pan-European corridors.

Croatia is classified by the World Bank as a high-income economy and ranks very high on the Human Development Index. Service, industrial sectors, and agriculture dominate the economy, respectively. Tourism is a significant source of revenue, with Croatia ranked among the 20 most popular tourist destinations. The state controls a part of the economy, with substantial government expenditure. The European Union is Croatia's most important trading partner. Croatia provides social security, universal health care, and tuition-free primary and secondary education while supporting culture through public institutions and corporate investments in media and publishing.