Two decades since fighting itself free of Yugoslavia, Croatia will become the 28th member of the European Union at midnight.
The accession comes against a backdrop of economic woes in the Adriatic republic and the bloc it is joining.
The country of 4.4 million people becomes only the second of the seven states carved from federal Yugoslavia to enter the EU, following Slovenia in 2004.
It represents a milestone in Croatia's recovery from a 1991-95 war to secure independence in which 20,000 people died.
The country's Adriatic coastline has since become a magnet for 10 million tourists every year, but a biting recession now in its fifth year has taken the shine off the celebration.
"We tend to underestimate this achievement," former foreign minister Kolinda Grabar Kitarovic told Croatian Nova TV.
"Let's not forget that, unlike many other countries, we had to fight for our very survival and international recognition only 20 years ago."
Croatia's accession marks the first expansion of the EU eastwards since 2007, when Romania and Bulgaria joined, and the "Big Bang" enlargement of 2004 when ten new members came aboard.
The global economic crisis has since strained EU solidarity, undermining popular support for the union and fuelling doubts over the wisdom of further expansion to the troubled states of the western Balkans.
In Zagreb, security was tight for the arrival of 170 foreign officials, including 15 heads of state, 13 prime ministers and the presidents of the European Commission and European Council, Jose Manuel Barroso and Herman Van Rompuy.
The main festivities kicked off at 11pm (9pm Irish time).
Serbian President Tomislav Nikolic, once a fierce advocate of the "Greater Serbia" ideology that helped fuel the wars in Croatia and neighbouring Bosnia, was also present, underscoring the change the region has undergone since the 1990s.
The ex-Yugoslav republics of Serbia, Bosnia, Macedonia, Montenegro and Kosovo are still years away from EU accession.
Revellers began pouring into Zagreb's central Ban Jelacic square, where EU flags flew from a stage.
Croatians voted in favour of joining the EU in a 2012 referendum, but the bloc's crippling debt crisis and rising youth unemployment have many questioning the wisdom of accession.
To join, Croatia has gone through seven years of tortuous and often unpopular EU-guided reform.
It has handed over more than a dozen Croatian and Bosnian Croat military and political leaders charged with war crimes to the UN tribunal for the former Yugoslavia.
It has sold shipyards, steeped in history and tradition but deep in debt, and launched a fight against corruption that saw former prime minister Ivo Sanader jailed.