Both Bulgaria and Romania had opportunistic reasons for entering the war, and settled for the alliance which could offer them more spoils and looked more likely to win at the time that they made their decision.

The summer of 1915 was the "Bulgarian summer" as both alliances jostled to swing Bulgaria to their side. An Entente-aligned Bulgaria would have helped the Gallipoli invasion by providing a direct threat to Constantinople. At the end of the day, however, the Entente simply could not meet Bulgaria's demands for Serbian territory in return for attacking the Ottomans. The Entente was unwilling to throw their ally under the bus, but the Central Powers could promise Bulgaria what it wanted. Moreover, the Central Powers were in the ascendant that summer, having smashed through Russian lines in the East, throwing the Tsar's armies into the "Great Retreat," while the Western Allies struggled to come ashore at Gallipoli and were unable to make any consequential gains on the Western Front. For Tsar Ferdinand and the Bulgarian government, the choice was not very difficult. The Ottomans sealed the deal by ceding a hefty piece of Eastern Thrace to Bulgaria, which meant there was no direct threat from Constantinople. And while the Ottomans were indeed the ancient enemy, Serbia and the rest of the Balkan states were the more recent one, having defeated an expansionist Bulgaria in the Second Balkan War of 1913 and left it nursing a grudge.

Romania sided with the Allies out of equally blatant opportunism. King Carol I was a Germanophile and declared neutrality in 1914. However, he died in October of that year and the throne passed to his less conservative nephew Ferdinand I, which tipped the balance of power in Bucharest towards the Liberals and their Prime minister Ion Brătianu, who saw joining the Entente as a way of annexing Transylvania and Bessarabia with their large Romanian populations from the Austro-Hungarian Empire. The Allies coveted an alliance with Romania, whose large army could turn the balance of the Eastern Front. Brătianu knew this and played his cards to get as many concessions from the Entente as possible, as well as huge shipments of war material, before pledging his support in August 1916. Although geographically Romania was flanked on three sides by the Central Powers, that did not seem especially dangerous at the time. The late summer of 1916 was the height of the Brusilov Offensive, one of the the most dramatic Allied victories of the war, which had absolutely smashed through the Austro-Hungarian Army on the Eastern Front. The alliance seemed like a diplomatic coup for the Allies, as well as for Bucharest. In Berlin, Romania's alliance with the Entente caused a panic, with the Kaiser confiding to a friend that "the war is lost." It was the last straw for the government's confidence in Chief of the General Staff Erich von Falkenhayn, whose Verdun offensive had turned into a bloodbath and who now resigned his position, to be replaced by Paul von Hindenburg and his lieutenant Erich Ludendorff.

Unfortunately for Brătianu, his war plan was quickly revealed to contain a lot of wishful thinking The Romanian Army attacked Transylvania, but was stopped within two months by hastily-organized German and Austrian defenders. Although the Romanians outnumbered their enemies, movement through the Carpathian mountains proved to be far more difficult than anticipated, and experienced Central Powers troops made the Romanians pay for using outdated tactics. Incredibly, no one in Bucharest seemed to have anticipate that the Bulgarians to the south would cross the border, despite the fact that for months it had been increasingly obvious that Romania was preparing to join the Entente. And worst of all, Romania joined the war just as the Brusilov Offensive began to run out of steam, giving the Germans breathing room to organize a counter-attack from three sides into Romania. Because of all this, Romania's war turned into a disaster in a matter of months. In fact, the Allies realized too late that Romania had probably been more useful as a neutral nation, as its entry into the war allowed the Germans another way to strike at Russia's over-extended Eastern Front. The German-led counterattack began in September, and by December Bucharest had fallen.

Ultimately, I would argue that neither Bulgaria or Romania significantly affected the outcome of the Great War. Romania's entry actually made things worse for the Allies, and although the conquest of Romania was no cakewalk, Bucharest fell in a short time, which left the Russians with a gaping hole in the Eastern Front that needed to be defended. Bulgaria was a more useful ally, and its entry into the war provided the death-blow to Serbia in 1915, but at the end of the day even the "Prussia of the Balkans" relied heavily on German aid, despite the mobilization of almost its entire population to the war effort. The Bulgarian Army spent the bulk of the war manning the static Salonika Front, where malaria was a much heavier killer than British or Greek shells. Bulgaria's capitulation in September 1918 marked the beginning of the collapse of the Central Powers, but it was hardly the case that Bulgaria or Germany's other allies were propping up the Kaiser, and that knocking them out caused the Germans to be defeated. It was rather the opposite - Germany propped up its allies throughout the war, and their defeats in 1918 reflected the collapse of German power.

That said, the war affected the populations of Romania and Bulgaria heavily, and their armies participated in many battles on the scale of the Western Front, such as the Romania defense of Mărășești in 1917, or the Bulgarian victory over the British at Lake Dorian in 1917.