"One year on, Crimea still is Ukraine, and it is our joint responsibility with the rest of the world to undo the injustice de facto and de jure - to make the aggressor go. Sooner or later Crimea will return to where it belongs, and our joint duty is to make it [happen] sooner - out of respect of the rights of our citizens, to international law and for the sake of safeguarding global security," Poroshenko wrote.
Poroshenko said that one year ago, the Ukrainian territory of Crimea was illegally annexed by Ukraine's neighbor and partner at the time, Russia.
"I myself witnessed the illegal and shameful occupation, and I will never forget or excuse it. When I visited the Crimean capital of Simferopol to help negotiate a settlement one year ago, I saw many 'little green men', who were in fact heavily armed professional soldiers. Although they were masked and disguised, with their uniforms and markings altered, it was clear that every command for the occupation had come from one source: the Kremlin," Poroshenko said.
He also said that Russian aggression in eastern Ukraine has killed more than 6,000 people and has turned Donbas into a ruin.
"Despite a renewed ceasefire that I signed in Minsk last month, periodic violence continues," Poroshenko said.
He said the annexation of Crimea was not only an assault on international law, it also robbed Ukrainian citizens of the right to live in their own state.
"Moscow quickly exported its strong-arm rule, cracking down on dissent, the media and access to information. Those who refuse to accept Russian citizenship are considered foreigners, they are given no protection against deportation and are denied access to basic services," he said, pointing to the Crimean Tatars who have suffered the most under the occupation.
Poroshenko said that on March 27, 2014, 100 United Nations member states voted in favor of a resolution affirming their support for the territorial integrity of Ukraine and recognition of Crimea as a part of Ukraine.
"We remember and appreciate this display of international solidarity in our time of need, and we believe that the Crimean people will regain their native land," he said.