Since ancient times, politics had always been filthy. Don't be fooled by the Entente and the Alliance fighting one another to the death on the battlefield—in private, their mutual contacts had never ceased, and some under-the-table dealings were still going on.
Between nations, allies and enemies had always been separated by only a thin line. As long as the interests were great enough, allies could become enemies, and enemies could become allies!
The most typical examples were Britain and France, hereditary enemies, and Britain and Russia, mortal foes, becoming allies under the pull of shared interests, while Britain and Germany, who had been negotiating an alliance all along, suddenly became enemies.
Even a year before their formal alliance, if anyone had said that Britain and France, or Britain and Russia, could form an alliance, everyone would have taken it as a joke!
When Ferdinand had held back during the attack on the Austro-Hungarian Empire, that had not been something Bulgaria could decide unilaterally. In fact, the Austro-Hungarian Empire had actively cooperated as well!
The clearest example was that after suffering losses at the outset, the Austro-Hungarian Empire changed its strategy and decisively let Bulgaria go, choosing instead to duke it out with Russia. Certainly, Bulgaria's terrain, easy to defend and hard to attack, was a factor, as was the enormous pressure on their Eastern Front while the Russian Bear was attacking. But more importantly, Bulgaria and Austria-Hungary had already reached an unspoken understanding!
The purpose of the behind-the-scenes deal was actually quite obvious: 1. Russia posed too great a threat to the Austro-Hungarian Empire. Even if Austria-Hungary did not attack the Russian Bear, the Russians would not spare them anyway;
2. A strong Russia did not serve Bulgaria's interests. Especially after Bulgaria had broken free of Russian control following the