Kurdistan (Kurdish: کوردستان) is a region in Western Asia divided between Turkey (Northern Kurdistan), Iraq (Southern Kurdistan), Syria (Western Kurdistan) and Iran (Eastern Kurdistan), inhabited by Kurds, as the name implies. Kurds are most closely related to the Iranians, being an Iranian ethnic group like the Persians, Pashtuns and Tajiks and formerly part of the Persian Empire, but they have their own separate culture, languages, and primarily adhere to Sunni Islam as opposed to Shia Islam, and Turkey has the largest Kurdish population of the four countries at about 14.3 to 20 million. Yet, despite having their own unique culture and identity from the rest of the Middle East and playing a major role in the battles against ISIS in Iraq and Syria, they are a stateless nation, and at around 36.4 to 45.6 million people, they are the largest ethnic group in the world without their own country, and the fourth largest ethnic group in Western Asia after Arabs, Persians and Turks.
So why don't they have their own country? Well, they've tried to create an independent state before, but none of their attempts succeeded. Let's begin with World War I, when the Ottoman Empire dissolved (which Kurdistan had been integrated into during the Ottoman-Persian Wars). Kurds were very unhappy with the radical secularization (the Kurds being strongly Muslim), centralization of authority and rampant Turkish nationalism that was taking place in the country at the time, due to it threatening to marginalize them as well as the power of local chieftains and Kurdish autonomy. Around 700,000 Kurds had been forcibly deported by the Young Turks during the Armenian genocide, and almost half of them died. Some Kurdish groups sought the confirmation of Kurdish autonomy and self-determination in the Treaty of Sèvres, but Kemal Atatürk prevented it in the aftermath of World War I. Kurdist revolutions were suppressed by the Turks and Iranians. British-backed Kurds did managed to declare independence and create the Republic of Ararat in Eastern Turkey on October 28th, 1927 or 1928, but it was quickly defeated and taken over by the Turks in September 1930. A Kingdom of Kurdistan existed in Iraq from September 1922 until July 1944, being defeated by the British and given back to Iraq with the provision for special rights for Kurds. A Soviet-sponsored Republic of Mahabad briefly existed in Iran in the 22nd of January, 1946, but quickly fell later that year.
Following several large scale Kurdish revolts in Kurdistan in the 1920s and 1930s, Turkish Kurdistan was put under martial law and many Kurds were displaced, with the Turkish government also encouraging resettlement of Albanians from Kosovo and Assyrians in the region to change the make-up of the population. Needless to say, the Kurds weren't very happy at Ankara for a long time after this, and the 1960 Turkish coup d'état halting their move towards integration in the Turkish government in the 1950's didn't help either. Eventually, the PKK (Partiya Karkerên Kurdistanê), also known as the Kurdistan Workers' Party, was formed in the 1970s by Kurdish nationalists influenced by Marxist political thought, and has been involved in an armed struggle against the Turkish state since 1984 for cultural and political rights and self-determination for the Kurds, with open war between the PKK and Turkish military occurring from 1984 to 1999, the Turkish government banning the words "Kurds", "Kurdistan" and "Kurdish" and prohibiting the Kurdish language, and the Turkey committing thousands of human rights abuses. Even today, Kurds in Turkey are still not allowed to get a primary education in their mother tongue and don't have a right to self-determination.
Meanwhile, in Iraq, a war started between Kurds and Iraqis in the 1960s, and during the IranIraq War, a civil war broke out due to the regime's implementation of anti-Kurdish policies, which resulted in the mass murder of hundreds of civilians, the wholesale destruction of thousands of villages and the deportation of thousands of Kurds to southern and central Iraq by the Iraqis, with the most infamous attack by the Iraqis being in Halabja in 1988, which killed 5000 civilians instantly. The Iraqis recaptured most of the Kurdish areas of Iraq after the collapse of the Kurdish uprising in March 1991 and more than a million Kurds to the Turkish and Iranian borders, with 20,000 dying in the process. Eventually, following UN intervention, the Kurds managed to capture Erbil and Sulaimaniyah and establish the Kurdistan Regional Government. The area under control of Peshmerga was then expanded, and the Kurds now effectively control Kirkuk and parts of Mosul, with the authority of the KRG and legality of its laws and regulations being recognized in the articles 113 and 137 of the new Iraqi Constitution ratified in 2005, and the regions of Erbil and Sulaimaniya being unified.
In Syria, Kurds make up the largest ethnic minority, and the Syrian government have employed techniques to suppress their ethnic identity like various bans on the use of the Kurdish language, refusal to register children with Kurdish names, the replacement of Kurdish place names with new names in Arabic, and the prohibition of businesses that do not have Arabic names, Kurdish private schools and books and other materials written in Kurdish. Around 300,000 Kurds were deprived of any social rights, in violation of international law, though the Syrian government eventually promised to tackle the issue and grant Syrian citizenship to them to avoid further demonstrations and unrest. Eventually, because of the Syrian Civil War, the Kurds were able to take control of large parts of Syrian Kurdistan from Andiwar to Jindires, and started the Rojava Revolution in 2013, with the Democratic Union Party having plans to establish a new constitution for the de facto autonomous region.
Needless to say, Kurds have had it rough. The Iranian government have at least treated them better than the Turkish, Iraqi or Syrian governments, however, due to their shared history and being cultural and ethnolinguistical relatives, with many Iranian Kurds and their leaders not wanting a separate Kurdish state, Kurds being well integrated into Iranian political life and the Kurdish language being used more today in Iran than any other time since the Iranian Revolution, though of course the Iranian government are opposed to any suggestion of Kurdish separatism, and have run into problems with Kurdish nationalists, most notably in the Iran crisis of 1946 and with the PJAK.
The Kurdish flag