Romania made a significant contribution to the Balkan front of the Russo-Turkish War. They committed about 120,000 men to the Russian side of the war and played an important role in one of the decisive battles on that front. It was also decisive for Romania itself, which achieved de jure independence as a result of the war (the war is called the War of Independence in Romania for that reason).

Romania was in a tricky position as the Balkan Crisis accelerated in the mid-1870s, because although it had been de facto independent since 1866, it still hadn't received formal recognition of its independence, and was still forced to pay tribute to the Ottoman Empire. Romania initially tried to stay neutral in the crisis, to avoid committing itself to a disadvantageous position. They feared Russian expansionism, and were skeptical of allowing the movement of Russian troops across Romanian territory for that reason. Initially, Russia wasn't interested in Romania as a military ally because they didn't want to give Romania a seat at the table during the postwar negotiations, which would be Romania's primary motivation for participating in a war against the Turks. A particular sticking point was the status of southern Bessarabia, which had become part of Moldavia (one of the two principalities that made up Romania at that time) at the end of the Crimean War; Romania wanted formal recognition that this was Romanian territory, while Russia wanted it back. Negotiations between Romania and Russia continued until late fall 1876, when a provisional agreement on military and political matters was reached.

A conference of the Great Powers took place in Constantinople in December 1876 and January 1877 in an attempt to resolve the crisis, but talks broke down, and the Ottoman Empire re-asserted its position that Romania was still Ottoman territory. This convinced the Romanians that their only choice for recognition of full independence was to cooperate with Russia. The two sides reached a tentative agreement on military and political terms in April 1877, but it wasn't ratified until after Romania had mobilized and hostilities had commenced on 24 April. Romania formally declared war on the Ottoman Empire on 12 May, and formally declared independence on 21 May.

Romania's initial task was to maintain pressure on Turkish forces along the Danube. Russia originally did not envision a role for Romania in the invasion of Bulgaria, which began in July 1877. However, although Russian forces crossed the Danube largely unopposed, they ran into a roadblock at the heavily fortified city of Plevna (Pleven). The Russians launched two direct frontal attacks against the city, which were defeated by the entrenched Ottoman forces led by Osman Pasha, and as losses mounted, Grand Duke Nicholas petitioned Prince (later King) Carol I to send Romanian troops to reinforce the Russian ranks.

On 28 July, Carol agreed to the Russian request, and the Romanians sent over 40,000 men across the Danube to join the fighting at Plevna. During the next attack on the city in September, Romanian forces succeeded in capturing and holding the redoubt at Grivitsa in the face of an Ottoman counterattack, while Russian forces were pushed back from the 13 other posts they had occupied during the attack. The Russo-Romanian forces still couldn't break through the Ottoman lines, and by October, they instead opted to encircle and starve out the Ottomans rather than continuing to bash their heads against the walls.

Eventually, by December, the Ottoman forces were almost out of supplies, and they were forced to attempt a breakout. By this time, they were massively outnumbered (almost 5 to 1) and were unable to break the encirclement. They were forced to surrender on 10 December 1877; Osman Pasha handed his sword over to a Romanian officer, Colonel (later General) Mihail Cerchez. Pleven was the decisive battle on the Balkan front of the war, and once the Turks had surrendered Pleven, their hopes for victory in the west were pretty much gone (although it did slow down the Russian advance enough that they couldn't reach Constantinople).

Romanian involvement was more limited in the final weeks of the war, as they were mostly reduced to guarding the Russian flanks as they pressed deeper into Bulgaria. Romania also fought several naval engagements against Ottoman ships on the Danube, including a battle in late May 1877, in which the Romanian torpedo boat Rândunica sank the Ottoman ironclad Seyfi near the town of Măcin in the Danube delta region.

Romania wasn't invited to the talks that led to the armistice between Russia and Turkey in January 1878, which they viewed as a betrayal by the Russians. Russia also still seemed intent on excluding the Romanians from the postwar peace talks. The Romanians were further outraged by the terms of the Treaty of San Stefano, which formally ended the war; the treaty recognized Romania's independence, but it awarded southern Bessarabia to Russia, in exchange for the territory of Dobruja, which gave Romania access to the Black Sea. This settlement was confirmed at the Congress of Berlin later that year (from which Romania and the other new Balkan states were largely excluded), which formally recognized Romania as an independent state, but allowed the territorial provisions of the Treaty of San Stefano to stand. (Romania's independence wasn't fully recognized until 1880, when a new constitution that allowed the emancipation of the Jews had been approved.)

Source: Keith Hitchins, Rumania: 1866-1947 (Oxford UP, 1994).