Expectations and surprises
General Mihail Savov
During June of 1913, the month that followed the signing of the London treaty, everyone could see that a second war was inevitable. There were last-minute overtures (a meeting between the Bulgarian and Serbian prime ministers, Danev and Pašić, at the border of their countries; Venizélos’ visit to Sofia on the way home from London) but nothing came of them. Bulgaria, freed by the settlement from having to keep most of its troops on the Ottoman frontier, transferred them to positions confronting the Serbs and Greeks in Macedonia, with a few sent to guard the Romanian frontier in the north. Toward the end of the month, episodes of sniping and raiding were beginning to break out between the former allies.

On the night of June 29th, the commanders of two of Bulgaria’s five armies received this order, issued by General Mihail Savov and signed by Tsar Ferdinand, to whom the general was deputy commander-in-chief:
So that we do not ignore the Serbian attacks, which would reflect badly on our army’s morale, and to press the enemy further, I am ordering you to attack the enemy by the most energetic means along the entire line, without completely disclosing your strength and without becoming involved in a prolonged battle. [Order to the 4th Army, quoted by Hall; the 2nd army was ordered, in the same terms, to attack the Greeks].
Tsar Ferdinand I of Bulgaria
To the Greeks and Serbs, as well as the rest of Europe, this action—regardless of how the orders (which of course they didn’t see) were worded—looked like a full-scale offensive, launched without any warning or declaration of war. Historians tend to think, however, that this was not what the Bulgarians wanted; it was intended more as a warning demonstration of their armies' might. The generals persuaded the tsar to approve the order, but the attack came as a complete surprise to the government, which wasn’t consulted.

Hall quotes another historian of the Balkan Wars:
The Bulgarian attack of June 29/30 is explicable only as part of a policy of arbitration and not as one directly opposed to it. Military action was thus intended as a means of strengthening Bulgaria’s position in the settlement which was to come through the mediation of Russia. [E. C. Helmreich, The Diplomacy of the Balkan Wars 1912–13.]
It’s hard to disagree with Hall’s comment: “Even [though Bulgaria’s other three armies were not ordered to attack], given the extreme tension and bellicosity that existed on both sides, the Bulgarian idea of fighting to improve a negotiating position was, at best, ill-advised.” The perception that Bulgaria intentionally began the war with a sneak attack was generally accepted, and—when the time came to negotiate the next peace—helped Greece and Serbia appear as the victims of unprovoked aggression and therefore the rightful winners of the war.

Both Greeks and Serbs had, of course, been expecting a Bulgarian move of some kind, and both were able to mount a strong resistance and counterattack. On the Bulgarian side there was great confusion over exactly what their armies were doing. The government, totally out of the loop, demanded that the troops stop shooting and return to their bases. The military high command, having authorized something less than a full offensive, was alarmed by the results and issued the more or less the same order. For the first four days of the war, many Bulgarian army units didn’t know whether they were supposed to be fighting or not.

Neither Greeks nor Serbs were in any such doubt, and the success of their counterattacks owed a good deal to the confusion on the Bulgarian side, which wasn’t overcome until July 4. By that time Bulgaria had lost quite a lot of Macedonia.

King Constantine led a Greek army 121,000 strong against the Bulgarian 2nd army, which was later described by its general as comprising 36,000 troops, 20,000 of these untrained. Hall believes that the general may have been minimizing the size of his force—he had, after all, lost the battle—but the 2nd army was certainly smaller than the Greek army’s estimate of 80–105,000. Wikipedia, citing a Bulgarian source, gives a number slightly above 75,000. The Bulgarian high command, still harboring illusions about the military incompetence of Greece, had deployed most of its strength elsewhere.
Battle of Kilkis (artist's conception)
The 2nd army was stretched out along a line 120 miles long, and on the first day of the war the Greeks pushed the entire force rapidly northward. The Bulgarians fell back to prepared defensive positions near the Macedonian town of Kilkis, about 30 miles north of Salonika, where a battle was fought from July 1–3. It ended in what Hall calls “the most serious military disaster suffered by the Bulgarians in the Second Balkan War, and the greatest Greek success of both Balkan Wars.” (It’s unlikely that the battle looked much like this colorful print, however.)

Certainly the Greeks owed a part of their success to superior numbers, but that isn’t the whole story. Small armies have sometimes defeated big ones when the spirit was willing. Bulgarian soldiers had done a lot of hard fighting in the first war, and by now they were more than ready to go home. Recognition of the general dissatisfaction in the ranks had made the high command eager to commence hostilities as soon as practicable, on the theory that the patriotic spirit generated in battle would restore their armies’ morale.

Nevertheless, Macedonia wasn’t necessarily as important to Bulgaria’s common soldiers as it was to their tsar, their generals, and their newspapers. Hall mentions a report in a junior officer’s diary that neither officers nor men were eager to fight a war over Macedonia. The officer quotes a soldier whom he had heard arguing that, since the Serbs and Greeks weren’t willing to give them the whole province, they should just divide it.

At Kilkis, the Greek infantry had to advance across an open plain commanded by Bulgarian artillery batteries whose fire was painfully accurate. The Greeks took heavy casualties and had to withdraw. Superior numbers didn’t make that experience any easier, but they did ensure that it would be repeated, as it was on the second day of the battle with no greater success. On the last day, a third attack—begun in the small hours of the morning—broke through the defenders’ lines, and by midday the town was in Greek hands and the Bulgarians were in full retreat.
King Constantine inspiring his troops
The Serbs had also succeeded in repelling the Bulgarians after their initial attack. But Bulgaria wasn’t beaten yet, and, in the second week of fighting, counterattacks pushed the Serbs back into a defensive position. King Constantine, however, was inspired by his victory at Kilkis (or perhaps by his temperament) to believe that a major victory was within his grasp. He was a good enough general on the tactical level, and he cut a fine figure on horseback, as this adoring portrait by the French artist-correspondent Georges Scott illustrates. But his ability to see the bigger picture seems to have been limited.

His army won another fight near Lake Doiran (now on the Greek-Macedonian border) and, led by its commander-in-chief, pursued the retreating Bulgarian 2nd army up the valley of the Struma River, well past any point that Greece had an interest in claiming. Venizélos called this to Constantine’s attention, but the king shrugged him off. As always, he was impatient with any attempt to impede a military action by taking political considerations into account. The only military action Constantine now had in mind was a decisive battle in which he and his soldiers would finally and completely crush the Bulgarians and march triumphantly into Sofia.

But this wasn’t fated to happen. For two weeks, the Greeks pushed northward up the Struma valley, chasing the 2nd army toward the city of Gornya Dzhumaya (in 1950 renamed Blagoevgrad after the founder of the Bulgarian Communist Party). Sofia lay another 60 miles farther on, but—given that expected decisive victory—anything seemed possible.

However, the Greek advance stalled well short of Gornya Dzhumaya in the Kresna Gorge, steeply walled by mountains. Constantine had reached the end of his lines of supply and communication, and his troops were exhausted. Proposals for an armistice were now being heard. Venizélos’ government was inclined to accept, and he visited Greek army headquarters on what’s now the Greek-Bulgarian border, to suggest this course to the king (presumably by field telephone or telegraph, as the king was with the army many miles to the north). But Constantine still held out for the big victory.
Greek artillery in the Kresna Gorge
The Bulgarians, unfortunately for this plan, had managed to regroup and concentrate their forces, and they’d won a couple of defensive battles against the Serbs, who dug in and pretty much halted their attack. That enabled Bulgaria to send more troops against the Greeks, who by July 29 were under attack by Bulgarian armies shooting down at them from both sides of the gorge. They had to struggle up the sides with their artillery, trying to get to a level where it could answer the Bulgarian fire effectively, but this was far from easy.

The Greek army was now in serious danger of being encircled and annihilated (and Constantine, as Hall points out, was at grave risk of suffering the same humiliation as Napoleon III of France, who was captured by the enemy on the battlefield.) His soldiers had little fight left in them.

The king was compelled to see the light at last, and he sent Venizélos, then in Bucharest, the following telegram, quoted by Hall: “My army is physically and morally exhausted. In the light of these conditions I can no longer refuse the armistice or suspension of hostilities. Endeavor to find some way of securing a suspension of hostilities.” (By ‘morally exhausted,’ the king didn’t mean that the army was threatened with an outbreak of depravity—he meant that its morale couldn’t be much lower, which is certainly understandable.)

Even Venizélos’ diplomatic skills might not have been sufficient to prevent a military disaster, but Greek honor was saved by the sudden and timely collapse of the Bulgarian war effort: Romanian cavalry units had showed up only seven miles from Sofia.
King Carol reviews his invading army
The Romanians, with plenty of encouragement from Greece and Serbia, had made good on their threats. Three weeks earlier, on July 10, they had declared war on Bulgaria, and a Romanian army of 80,000 men overran Southern Dobrudja the next day. But their government decided to take matters a good deal farther: On the 14th and 15th, 250,000 Romanian troops crossed the Danube at three points and entered northern Bulgaria, whose government decided to offer no resistance. Decided may be the wrong word; at that point there was practically nothing they could do. This photo on a Romanian postcard shows King Carol I reviewing his troops before they cross the river. (The old-calendar date in the caption works out to July 15 on the modern calendar.) I'm not sure how many Romanian troops were equipped with bicycles, but I do believe the cavalry was still using horses to get around.

Hall, pointing out that the Bulgarian military was just at that point beginning to recover its balance and show signs that it might yet be a match for its Serbian and Greek opponents, declares that “the Romanian thrust across the Danube was the decisive military act of the Second Balkan War.” Decisive is certainly the right word here—the Romanian action decided the outcome of the war and made Greece and Serbia (and of course Romania itself) the unchallenged victors.

Romania wasn’t alone in piling onto the collapsing Bulgarians: the Turks jumped in too. Intent on its planned conquest of Macedonia, Bulgaria had left few forces in the southeast, and Enver Bey’s troops recovered Edirne and occupied everything up to the old border of Bulgaria without firing a shot. Neither they nor the Romanians suffered a single combat casualty. But that doesn’t mean they paid no price: cholera had broken out in Bulgaria during the first Balkan war, and during the second, it killed 4,000 Turkish and 6,000 Romanian troops. (It took a much higher toll of Bulgarians: military deaths due to disease have been estmated at 19,000 in the first Balkan War and 15,000 in the second. Most of these deaths can probably be attributed to cholera.)