Just wanna point out that Bulgaria did not start a four-front war, they "only" attacked Serbia at first, hoping to crush them while standing on the defensive against Greece. That Romania and the Ottomans would get involved was not something Bulgaria planned for, or believed would happen.

The end of the First Balkan War left many of the warring parties, especially Serbia, feeling betrayed and fooled by the European powers, because of the Treaty of Berlin.

They had been locked away from the Adriatic Sea, and with that option lost to them they instead turned their eyes to Bulgarian Macedonia, and on 1 June 1913, Serbia and Greece signed a secret treaty.

The treaty stipulated that a Serbian-Greek border were to be drawn east of the Vardar river and splitting Macedonia west of the Vardar.

The planned division of Macedonia between Serbia and Greece was simple enough, the countries would keep the territory they conquered.

Bulgaria understood that an already unstable situation was quickly getting even more out of hand.

Demands for this question to be resolved on the battlefield grew louder and louder.

Especially Macedonian refugees in Bulgaria demanded war, and they threatened to kill politicians and other persons in power who opposed war.

Given the many assassinations (both attempted and succeeded) and the terror that could be linked to Macedonia, especially the VRMO ("Internal Macedonian Revolutionary Organization"), this was a threat that Bulgaria could not ignore.

But in any way, the Bulgarian military did not need any threats; they were already dead set on going to war.

The Bulgarian Chief of Staff Mihail Savov suggested that the army should either attack or demobilize, since discontent and threats of mutiny spread among the soldiers.

Developments during the Spring of 1913 did not sit well with Russia, especially not the threatening war between the former allies of the Balkan League.

The Russian Tsar Nikolai II appealed personally to King Ferdinand of Bulgaria and King Petar of Serbia, telling them to solve their problems peacefully and avoid war.

Both regents responded quickly and positively, leading to the Russian Foreign Minister Sergej Sazonov to invite Serbian Prime Minister Nikola Pašić and his Bulgarian colleague Stojan Danev to St. Petersburg.

The Russians wanted Serbia and Bulgaria to present their territorial demands, and then allow Russia to mediate, and the Russians also took this chance to urge the Balkan countries to demobilize.

For Bulgaria, a swift resolution was important.

The repeated warnings from Mihail Savov that the army was close to mutiny scared the Bulgarian government, so they requested that Russia resolve the "Macedonian question" within 8 days.

This (in Russian eyes) odd request proved to be a diplomatic disaster.

Sazonov was furious, he interpreted the Bulgarian request as an ultimatum, and the Russians wrongfully believed this Bulgarian attitude meant that they had signed a secret pact with another Great Power, most likely Austria-Hungary.

Sazonov demanded to speak with the Bulgarian ambassador in Russia, Stefan Bobčev, who he then began screaming and yelling at because of an alliance that did not exist.

According to Sazonov, both Serbia and Bulgaria had acted like idiots, and finally he explained that all previous treaties and agreements between Russia and Bulgaria were no longer in effect.

To the Bulgarians, the fury of Sazonov confirmed that Russia was not unbiased, but had already signed an agreement with Serbia (something that had not happened).

The Bulgarian public demanded action, and on 29 June 1913 Bulgarian troops attacked Serbian border posts, thus beginning the Second Balkan War.

Even though both King Ferdinand and Prime Minister Danev knew about a coming offensive, the government weren't told about the Bulgarian attacks until after they had begun.

A majority of the Bulgarian troops had been stationed along the borders to Serbia and Greece, and the plan was to quickly surround and crush the Serbian army in Macedonia.

This demanded 4 out of 5 Bulgarian armies, but to the surprise of the military, the Bulgarian government refused to follow this strategy.

The Bulgarian government were unwilling to show the world that war had come, and instead wished to potray this as a minor problem between two alliance partners, and because of that they only released 2 of the armies for the pincer movement against Serbian troops.

The Bulgarian attack on 29 June surprised the Serbs, but they quickly came to terms with the difficult situation, and just 2 days later, on 1 July, Serbian General Radomir Putnik ordered a counterattack and a day later, the Bulgarian 4th Army was in full retreat.

The counterattack continued for a couple of days, until the Bulgarians decided to change tactics; they struck the Serbs from the south and appeared behind the Serbian army.

Soon two Bulgarian armies and the Serbian and Greek armies were caught up in the fight.

Slowly, the Bulgarians lost the initiative; Greeks and Serbs won more and more often on the battlefields, and Bulgaria was forced to send two more armies into the fight, but it was not enough, the Bulgarians were forced on the defensive.

The mountainous terrain of Macedonia made offensive actions difficult, by the middle of July the front had stabilized.

However, by the end of July, a strategic turn of events led to a dramatic change. Romania began mobilizing on 5 July and declared war on Bulgaria on 10 July.
Ottoman forces advanced into Bulgarian Thrace 15 July, captured the strategically important Adrianopel/Edirne on 23 July and from there continued into Bulgaria.
Caught between Romania and the Ottoman Empire, and with their forces occupied in Macedonia fighting Serbia and Greece, Bulgaria was forced to surrender.