20th Century
After the World War I, Bosnia and Herzegovina became part of the Kingdom of Serbs, Croats and Slovenes which later transformed into the Kingdom of Yugoslavia. The Serb monarchy, being one of the victors of the World War, sought Croat and Slovene political parties as their partners when forming the country. MNO, reformed into the Yugoslav Muslim Organization (JMO), dropped the pursuit of Muslim national identity and focused on protecting the religious and existential issues of Muslims through coalescing with other parties, sometimes even with the Serbian parties such as Nikola Pašić's People's Radical Party and Milan Stojadinović's Serbian Radical Party.
In the 1921 census, only Serbs, Croats and Slovenes were recognized as native nations or "tribes", and these were the only available options for ethnicity. The result was that a large number of Bosniaks simply left the field for ethnicity blank. This phenomenon, labeled nonethnical element (nenarodni element), was a topic of heated debate amongst scholars and politicians for years to follow. Some of them argued that the nonethnical element were descendants of the Turkish occupier and as such should be expelled. Nevertheless, thanks to the helpful influence of JMO, there were only isolated incidents of oppression against Bosniaks.
This political void was quickly filled with a number of opposition parties which recognized Muslims as a separate nation. Among them was the Communist Party of Yugoslavia, as a document from the 1930s reveals. It's no coincidence that a large number of Bosnian Muslims joined the Communist Party, and later the partisans, many of them becoming prominent political leaders and commandants.
During the World War II, the authorities of the Nazi-puppet Independent State of Croatia tried to ally with the Bosniaks whom they considered to be "Muslim Croats" against the Serbs and other "undesirables". As a token, the Artists Gallery museum (by Ivan Mestrovic) in Zagreb was furnished with minarets and ceded to be used as a mosque.
The Declaration of the State Anti-Fascist Council of National Liberation of Bosnia and Herzegovina (ZAVNOBiH), issued on November 25 of 1943 by the partisan government, is widely considered to be the constitutional basis of the modern Bosnia and Herzegovina. This document uses essentially the same wording as the 1910 Constitution. Furthermore, the Resolution of ZAVNOBiH states: "Today, the nations of Bosnia and Herzegovina, through their only political representative - the ZAVNOBiH, desire that their country, which is neither Serb, nor Croat nor Muslim, but Serb as well as Croat and Muslim, should be the free and united Bosnia and Herzegovina in which the full equality, legal and otherwise, of Serbs, Muslims and Croats will be guaranteed".
Unfortunately, this declaration was broken as soon as World War II was over, as the Constitution of the Federal People's Republic of Yugoslavia (later Socialist Federal Republic of Yugoslavia) mentioned Serbs and Croats, but not Muslims, as the native nations (narodi). In the Yugoslav census of 1948, 90% of Muslims in Yugoslavia declared themselves as "nationally undetermined". Furthermore, many who registered as Serbs or Croats did so largely out of societal and economic pressure. When the "Yugoslav, nationally undeclared" option became available in 1953, 900,000 people registered as such.
With a weakening of Serb dominance in Bosnian communist leadership, the door opened up for a new national identification. Finally in the 1961 Yugoslav census, the "Muslims in the ethnic sense" option first appeared. By 1963 Muslims were listed in the Bosnian constitution alongside Serbs and Croats. Finally, in 1968, "Muslims" with a capital M was adopted as the term for a member of a nation rather than "Muslims" as adherents to Islam.
The decision wasn't greeted without debate among communist leadership, but Bosniaks had made themselves clear. "Practice has shown the harm of different forms of pressure," read a communique issued by the Bosnian Central Committee, "from the earlier period when Muslims were designated as Serbs or Croats from the national viewpoint. It has been shown, and present socialist practice confirms, that the Muslims are a distinct nation".
From then until the Yugoslav wars, Bosniak national identity continued to develop with two different philosophies forming. These breakthroughs in the 60s were not carried out by religious Muslims (in fact, they were headed chiefly by secular Muslim communists) but in the following decades two separate schools of thought emerged. The first, was a secular "Muslim Nationalism" (as supported by people such as Hamdija Pozderac), and the second was a separate revival of Islamic religious belief (a reaction to communist sponsored secularism and advocated by people such as Alija Izetbegović). The effects of these two separate ideas on what exactly Bosnian Muslims are that have also occasionally clashed can be seen to this day.
In September 1993, the Congress of Bosniak Intellectuals re-introduced the historical ethnic name Bosniaks instead of the previously used Muslim by nationality. Other ethnic groups (Serbs and Croats) objected to the name as a ploy to monopolize the history of Bosnia and make them seem to be foreign invaders (see History of Bosnia and Herzegovina). The term in itself means Bosnians and is an archaic term that was once used for all inhabitants of Bosnia regardless of faith. Bosniaks counter by pointing out that Bosniak is the historical term for their nation, and that had they truly wanted to "monopolize" Bosnian history it would have been far easier to use the "newer" version "Bosanci" which, however, has a somewhat different meaning.
Since the 1990s, the name has been adopted outside of Bosnia itself, onto the Slavic Muslim population of other former Yugoslav republics such as Serbia and Macedonia. It allows a Bosniak/Bosnian distinction to match the Serb/Serbian and Croat/Croatian distinctions between ethnicity and residence.
Read more about this topic: Bosniak History