Since the conquest of Serbia and Montenegro there has been relatively little fighting in the Balkans. An Allied force has established itself in Salonika in northern Greece, but it is too small to attempt the liberation of Serbia. The Bulgarians have been keen to attack Salonika, but the Germans have forbidden this, fearing that this would bring the Greeks into the war on the side of the Allies.
The one Balkan country that has remained outside the war so far is Romania. However the early successes of the Brusilov Offensive in Galicia have convinced Romanian leaders that the Austria-Hungary is about to disintegrate. Now the Romanians are secretly negotiating an entry into the war on the side of the Allies. The prize for them will be Transylvania, a Hungarian region with a large Romanian population.
The Central Powers have some inkling that Romania is intriguing with the Allies. In a reversal of previous policy, the Germans now authorise a Bulgarian invasion of northern Greece. The aim is to dislodge a Serbian force that has established itself in Greek Macedonia (after being shipped there from Corfu). In doing so they hope to intimidate the Romanians and deter their entry into the war.
In north-eastern Greece the Bulgarians enjoy great success, largely because they face no resistance. There are no Allied troops here, and the neutralist Greek government has ordered the Greek army not to resist the invasion. But in Macedonia things are different. After some initial successes the Bulgarian advance begins to run out of steam. The Serbs are reinforced and resupplied by rail, but the Bulgarians find it difficult to keep ammunition and provisions flowing to their men in this remote region. Alas, the offensive may now have the opposite of its intended effect, with the Romanians now beginning to think that the Bulgarians are little more than a paper tiger.
image source (Air War College: the Great War, by James Mowbray)