Primarily we must remember that the Shariah is a law.
Every law has its own terminology. As the entire human beings are identified with either a Muslim or a non-Muslim, the states too are identified as a Muslim state or a non-Muslim state.
In Shariah terminology, they are called Darul Islam and Darul Harb respectively. The word non-Muslim is applied to a person who believes in different faith and religion other than Islam. Similarly, the term non-Muslim state applies to various kinds of states. Deriving the meaning from the word Darul Harb, it is baseless and misguiding to say that a Muslim can not be a citizen of that state as it is obligatory for the Muslims to migrate from the non-Muslim state to a Muslim state. Shah Abdul Aziz has proved in his Fatwas that even the companions of the holy Prophet Mohammad (pbuh) had been the citizens of the Darul Harb.
Since 1955, there was criticism from different quarters on the Fatwa of Jamia Nizamia. The then Mufti of the Jamia Maulana Mohammad Maqdoom Beig wrote an exclusive article on Darul Harb.
The extract of the article is presented below. He quoted from Fatawa Alamgiri that:
Innal muraada be daarul Islami……tahta qahruhu
meaning Darul Islam is a state where the orders of a Muslim ruler are promulgated as it is under his control.
Darul Harb is a state where the orders of a non-Muslim ruler are promulgated and he rules the state. Same description has been presented in “Sharah Barjandi Mukhtesar al Waqaya”, page 539.
The question arises when a Darul Islam becomes a Darul Kufr or Darul Harb? There is difference of opinion between Imam Abu Hanifa and his two disciples as mentioned in Bada’e Assana’e Vol. 7, page 130. Abu Hanifa says Darul Islam becomes Darul Harb on fulfillment of three conditions.

1. There prevails the laws of infidelity.
2. It is contiguous to another Darul Kufr.
3. Muslims and non-Muslims do not continue trusting the former Muslim ruler.

Several Fiqh books like Barjandi, Durre Mukhtar, Fatawa Qazi Khan, Fatawa Sarajiya Khazanatul Mufteen, etc. contain the similar definition. Source of these definitions is Imam Mohammad who is quoted in Fatawa Alamgiri Vol. 2 as Darul Islam becomes Darul Harb on three conditions according to Imam Abu Hanifa :

1. The laws of non-Muslims are openly promulgated and the court cases are not adjudicated in accordance with the Islamic jurisprudence.
2. The territory is very close to another Darul Harb and in between them there is no land occupied by a Muslim states, i.e. it is not surrounded by the Muslim states
3. The Muslims as well as the non-Muslims are no more under the protection or trust of the former Islamic rule.

A Clarification about the third condition:

Inna la yahkumu feeha behukmul Islam:
Some scholars take the meaning of ‘Hukmul Islam’ as Ibadat, therefore, they say where there is freedom of ibadat like prayers, fasting, etc. that state is Darul Islam.
It is unequivocally decided that ‘Hukmul Islam’ means the court cases are judged under the Islamic jurisprudence, the orders and the judgments of the Qazi (Islamic judge) are implemented.
Hukmul Islam does not mean the Ibaadat but the implementation of the Islamic laws. If it were simply ibadaat, the whole world would have been called Darul Islam as there is no country where there are no Muslims who perform the obligatory five time prayers, fasting, etc.
In brief, Imam Mohammad and Imam Yousuf regarded the first condition only as enough to declare a Darul Harb with regard to the power and domination. But Imam Abu Hanifa considered the absolute domination as well as the three conditions as mentioned above.

The above Fatwa is dated: 6th Rabiul Awwal 1410H – 7th October 1989 and is signed by:
Maulana Khaleel Ahmed, Shaikhul Fiqh and Mufti Jamia Nizamia, Maulana Mohammad Waliullah, Shaikhul Aqa-aed, Maulana Mohammad Khaja Shareef, Shaikul Adab, Maulana Mohammad Abdul Jaleel, Shaikul Hadees and Maulana Ibraheem Al Hashmi, Shaikhut Tafseer

(Note:
Established in 1292H – 1875 A.D., Jamia Nizamia is regarded as one of the oldest Islamic Universities of South India. The last Nizam Kings Nawab Mir Mahboob Ali Khan and Nawab Mir Osman Ali Khan were the devoted patrons of the institution. Tens of thousands of Ulema, Mufti, Huffaz have achieved education so far. It enjoys the support and trust of majority of Andhra Pradesh, Karnatak and Maharashtra Muslims. Jamia Nizamia had produced hundreds of Qazis who worked as judges in the Muslim courts during the Muslim rule of Nizam kings. The books written under the patronisation of Jamia were taught in the schools, colleges and madarsas. At present too, the leading Qazis, Imams, Khateebs and scholars of Hyderabad belong to Jamia Nizamia.)

The critics of the Jamia may disagree with its views and Fatwas but no one can deny the services rendered to the Ummah by the Jamia. The views can be criticized but their sincerity to Islam and the purity of intention can not be challenged.