Democracy is a system that is contrary to Islam, because it gives the power of legislation to the people or to those who represent them (such as members of Parliament). Based on that, in democracy legislative authority is given to someone other than Allah, may He be exalted; rather it is given to the people and their deputies, and what matters is not their consensus but the majority. Thus what the majority agree upon becomes laws that are binding on the nation, even if it is contrary to common sense, religious teaching or reason. In these systems legislation has been promulgated allowing abortion, same-sex marriage and usurious interest (riba); the rulings of sharee‘ah have been abolished; and fornication/adultery and the drinking of alcohol are permitted. In fact this system is at war with Islam and its followers. 

Watch this Video to see the affect & reasons  https://youtu.be/G3OKeVaiaPk

Allah, may He be exalted, has told us in the His Book that legislative authority belongs to Him alone, and that He is the wisest of those who issue rulings and judge. He has forbidden the association of anyone with Him in His authority, and no one is better than Him in ruling. 

Allah, may He be exalted, says (interpretation of the meaning):

“So the judgement is only with Allah, the Most High, the Most Great!”

[Ghaafir 40:12]

“The command (or the judgement) is for none but Allah. He has commanded that you worship none but Him (i.e. His Monotheism), that is the (true) straight religion, but most men know not”

[Yoosuf 12:40]

“Is not Allah the Best of judges?”

[at-Teen 95:8]

An Islamic state is a state that has a form of government based on Islamic law. As a term, it has been used to describe various historical polities and theories of governance in the Islamic world.[1] As a translation of the Arabic term dawlah islāmiyyah (Arabic: دولة إسلامية) it refers to a modern notion associated with political Islam (Islamism).[2][3]

The concept of the modern Islamic state has been articulated and promoted by ideologues such as Sayyid Rashid Rida, Mohammed Omar, Abul A'la Maududi, Ayatollah Ruhollah Khomeini, Israr Ahmed, Sayyid Qutb and Hassan al-Banna. Implementation of Islamic law plays an important role in modern theories of the Islamic state, as it did in classical Islamic political theories. However, the modern theories also make use of notions that did not exist before the modern era.[1]

Today, many Muslim countries have incorporated Islamic law, wholly or in part, into their legal systems. Certain Muslim states have declared Islam to be their state religion in their constitutions, but do not apply Islamic law in their courts. Islamic states that are not Islamic monarchies are mostly Islamic republics.

https://en.wikipedia.org/wiki/Islamic_state