All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Dear brother, First of all you should know that, in principle, living in a non-Muslim country is forbidden. Imaam Abu Daawood narrated on the authority of Jareer Ibn Abdullaah, may Allaah pleased with him, he said that the Prophet said: "I am free from a Muslim who lives among polytheists". This is a sound narration.
So, living among them is forbidden as a general rule, because it has many negative effects on the religion of a Muslim. Those persons who face trials in their own countries or who fear that they may be imprisoned or killed in their countries, at that critical moment they may live in non-Muslim countries and Allaah will forgive them. The evidence for this rule is the migration of the weak Muslims from Makkah to Abyssinia because its king was a good and fair ruler who was not unjust to anyone.
Living in a non-Muslim country or migration to it is permissible only if a person cannot find a Muslim country where he can earn a livelihood and can practice his religion freely.
It is also permissible for Muslim scholars to live in non-Muslim countries for the purpose of Da'wah i.e., calling non-Muslims to Islam and teaching Muslims Sharee'ah rules, provided they they can do so freely and would not be subjected to trials or would have to compromise or leave their religion.
All the above mentioned details are related to the rule of Muslims living in non-Muslim countries. But the rule for applying and accepting citizenship of those countries is much restricted. A Muslim is not permitted to apply for citizenship of non-Muslim countries except in a dire need, such as not being able to practice his religion in his own country or fear of oppression or being in danger for his life or the threat of imprisonment or torture in his homeland and there is no Islamic country where he can live. In such circumstances one can apply for citizenship of a non-Muslim country. If he is obliged to take an oath for getting citizenship, at that moment one should try to allude to the words of oath as much as possible to escape their intended point.
However, if he is able to live harmlessly in the non-Muslim country without taking the citizenship then applying for citizenship is not permissible for him. It is forbidden for him to apply for citizenship of a non-Muslim country if he is simply aspiring to material comforts or financial gain.
Allaah Says (what means): {And never will Allaah grant to the disbelievers a way (to triumph) over the believers.} [Quran 4: 141].
No doubt, the believer who accepts the nationality of a non-Muslim country gives non-believers an advantage over him to force him to follow their rules and regulations.
Allaah knows best.