Question: Who was forced to readopt the Ottoman constitution of 1876?
About the article "Balkan Wars", section "Young Turk revolution".

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The Ottoman Constitution represented more than the immediate effect it had on the country.
Despite the latitude it gave to the sovereign, the constitution provided clear evidence of the extent to which European influences operated among a section of the Ottoman bureaucracy.
The constitution also reaffirmed the equality of all Ottoman subjects, including their right to serve in the new Chamber of Deputies.
The constitution was more than a political document; it was a proclamation of Ottomanism and Ottoman patriotism, and it was an assertion that the empire was capable of resolving its problems and that it had the right to remain intact as it then existed. 
  1876: Ottoman constitution of 1876, during the First Constitutional Era of the Ottoman Empire (1876–78)  1909: Revisions to the 1876 constitution during the Second Constitutional Era of the Ottoman Empire (1908–20)  1921: First constitution of the Republic of Turkey, 1921  1924: Revised constitution of 1924  1961: 1961 constitution; introduced an upper chamber of legislation, the Senate, to form a bicameral system  1982: 1982 constitution; abolished the Senate, reverting to a unicameral system  
The constitution of the Ottoman Empire was promulgated on 23 December 1876 .
It was prepared by the Young Ottomans (later Young Turks).
Midhat Pasha a notable member of the Young Ottomans, supported Abdülhamit II to throne in return for his approval to constitution.
Although Abdülhamit initially agreed to appoint Midhat as the grand vizier and form the constitutional parliament, soon he exiled Midhat Pasha.
In 1878 he abolished the parliament, and suspended the constitution indefinitely. 
After Abdülaziz's dethronement Murat was enthroned.
It was hoped that he would sign the constitution.
However due to health problems Murat was also dethroned after 93 days; he was the shortest reigning sultan of the Empire. 
The Ottoman Constitution was introduced after a series of reforms were promulgated in 1839 during the Tanzimat era.
The goal of the Tanzimat era was to reform the Ottoman empire under the guidance of Westernization.
In the context of the reforms, Western educated Armenians of the Ottoman empire drafted the Armenian National Constitution in 1863.
The Ottoman Constitution of 1876 was under direct influence of the Armenian National Constitution and its authors.
The Ottoman Constitution of 1876 itself was drawn up by Western educated Ottoman Armenian Krikor Odian, who was the advisor of Midhat Pasha.
It is important to discuss first the beginning of attempts at reform within the empire.
Under the reign of Sultan Selim III, there was a vision of actual reform.
Selim tried to address the military's failure to effectively function in battle; even the basics of fighting were been lacking, and military leaders lacked ability to command.
Eventually his efforts led to his assassination by the Janissaries.
This action led to Mahmud II becoming Sultan.
Mahmud can be considered to be the “first real Ottoman reformer,” since he took a substantive stand against the Janissaries by removing them as an obstacle (by killing them).
This led to what was known as The Tanzimat era, which lasted from 1839 to 1876.
This era was defined as an effort of reform to distribute power from the Sultan (even trying to remove his efforts) to the newly formed government led by a Parliament.
These were the intentions of the Sublime Porte, which included the newly formed government.
The purpose of the Tanzimat Era was reform, but mainly, to divert power from the Sultan to the Sublime Porte.
The first indefinable act of the Tanzimat period was when Sultan Abdülmecid I issued Edict of Gülhane.
This document or statement expressed the principles that the liberal statesmen wanted to become an actual reality.
The Tanzimat politicians wanted to prevent the empire from falling completely into ruin.
During this time the Tanzimat had three different sultans: Abdul-Mecid (1861 – 1876), Abdul-Aziz (1861-1876) and Murad V (who only lasted 3 months in 1876).
During the Tanzimat period, the man from the Ottoman Empire with the most respect in Europe was Midhat Pasha.
Midhat dreamed of an Empire in which “there would be neither Muslim nor non-Muslim but only Ottomans.” Such ideology led to the formation of groups such as the Young Ottomans and the Committee of Union & Progress (who merged with the Ottoman Unity Society).
These movements attempted to bring about real reform not by means of edicts and promises, but by concrete action.