Transylvania in Romania and Voivodina in Serbia. Both these territories have a high concentration of ethnic Hungarians. These regions belonged to the old Kingdom of Hungary before they were taken away by the Treaty of Trianon in 1920.
The conservative government of Hungary made it a priority to offer citizenship to ethnic Hungarians in neighbouring countries that were severed from the motherland due to Trianon. It should be noted that the vote to offer citizenship to ethnic Hungarians occurred several times in the Hungarian Parliament in recent years. The Hungarian Socialist Party voted against offering citizenship each time. The Hungarian conservative government finally passed the motion with their majority in parliament. The number of applications are still increasing and it seems that the plan to offer citizenship was a highly successful move by Hungarian conservatives.
For reactions in the neighbouring countries, click here.