KYIV -- The Association Agreement strengthening ties between Ukraine and the European Union entered into force on September 1, marking an end to four years of political drama surrounding the accord.
Ukrainian President Petro Poroshenko celebrated the development in a Facebook post.
He called it an "important step in confirmation of the future that we chose in the Revolution of Dignity," a reference to the street protests that drove President Viktor Yanukovych from power after he scrapped plans to sign the EU deal in 2013.
Kaja Tael, the Estonian ambassador to the EU, said that "it goes without saying that this is really a major upgrade in our relations" and that the two sides now have "the full range of all instruments at our disposal."
Tael, whose country currently holds the rotating EU presidency, noted that the European Union and Ukraine have "already been implementing parts of the agreement, some of them for years."
The landmark accord was originally meant to be signed in November 2013 by Yanukovych.
However, in a dramatic turnaround, Yanukovych walked away from the accord under pressure from Moscow, prompting massive protests that pushed him from power -- and into self-imposed exile in Russia -- in February 2014.
Poroshenko, the tycoon and pro-Western politician who came to power in the aftermath, signed the Association Agreement in June 2014.
But the deal -- which required unanimous EU approval -- remained unratified after 61 percent of Dutch voters opposed it in a citizen-driven, non-binding referendum in April 2016.
The Dutch government attempted to ease voter concerns in December by negotiating a legally binding supplement with the other 27 EU member states to underscore that the agreement would not grant Ukraine automatic EU membership or provide any guarantees of military aid.
Both the Dutch lower house and Senate voted in favor of the agreement earlier this year and ratified it in June.
All 28 EU member states formally endorsed the Association Agreement in July.
Many elements of the agreement have been provisionally applied since 2014, but the adoption of the accord will ensure closer cooperation between Brussels and Kyiv in areas such as foreign policy, justice, education, science, and technology.
After Yanukovych abandoned office and fled the country, Russia seized control of Ukraine's Crimea region and fomented separatism in eastern and southern Ukraine.
The Russia-backed separatists seized parts of the Donetsk and Luhansk regions, which border Russia, igniting a war that that has killed more than 10,000 people and continues despite a Western-brokered peace deal.
Russia's interference in Ukraine has led the United States, the European Union, and other countries to impose sanctions on Moscow, whose ties with the West have deteriorated to levels unseen since the Cold War.
With reporting by RFE/RL Correspondent Rikard Jozwiak in Brussels