The Struma sector extended along a very broad and marshy valley, and the enemy positions on the mountains east of the river were extremely strong. Moreover, a success in this direction would not have offered the chance of striking a blow at the heart of the enemy, such as was necessary to obtain really decisive results.
The vital centre of the enemy defences was the middle Vardar. From Uskub, from Veles, and from Gradsko, all their lateral communications spread out fan-wise. The Vardar railway put them in direct communication with the Central Empires and their supply centres. From Uskub there was a branch railway to Kalkandelen, whence a road, with a décauville along a part of it, descended into the plain of Monastir. From Veles and Gradsko all the troops in the area between Prilep and the Vardar, and to some extent also those immediately to the east of the river, were supplied. The enemy, however, counted on the very powerful defences, natural and artificial, on both sides of it, in the Demir Kapu gorge, to the west of Lake Doiran, and on the Beles range.
There was, however, another route which the enemy had not thought of fortifying, relying on its formidable natural defences. Between the Cerna and Nonte there is a group of high peaks rising from a wilderness of rocks around a basin hidden among the mountains. The area is called the Moglena, and it is here that the Moglenitza river has its source. The mountains of this group are among the highest in Macedonia; several summits, such as Floka, Kaimakchalan, Dzena, are over 2,000 m., the Mala Rupa is over 1,900 m., Sokol, Dobropolje, Vetrenik, Kuchkov Kamen and Kravitza are about 1,700–1,800 m. In the offensive of the autumn of 1916 the Serbs had conquered the Kaimakchalan range and the Moglena area, which was afterwards entrusted to the II Serbian Army. Since then the sector had been a peaceful one, except for the engagements in the spring of 1917. Voivod Michich had repeatedly insisted on the advisability of an offensive against the Dobropolje, as offering the best chances of success. General Sarrail had refused to trust the man who knew far more about Balkan warfare than he did. General Guillaumat, on the other hand, believed the plan to be possible, and began the preliminary studies for an operation in that direction. But more and heavier artillery was needed than the Armée d’Orient as yet possessed. Guillaumat, therefore, only executed the attack on the Srka di Legen. To General Franchet d’Espérey is due the merit of having taken up the plan once more, perfected it and finally executed it. As a result of his inspections along the front, he convinced himself that while the vital ganglion of the whole enemy organization was the middle Vardar, an attack on any of the four traditional sectors was, with the means at his disposal, impossible. From Mount Floka he studied the conditions of the ground with minute attention, and realized that a surprise attack in the Moglena area might succeed, because the Bulgars were not likely to expect it. It was necessary to break through in the centre, and once the breach was made to spread out in a fan-like formation to reach Gradsko and Demir Kapu, thus cutting the enemy’s communications and separating the XI German Army from the I Bulgarian Army. At the same time powerful diversive actions on the right and left wings (Doiran sector and Cerna loop) would nail down the enemy and prevent them from sending reinforcements to the centre.
To carry out this plan it was indispensable to concentrate a mass of powerful artillery in the central area, have strong reserves, and at the same time hold the vast line throughout the whole of its length. The scheme appeared impossible owing to the absence of one of these requisites—the reserves. But General Franchet d’Espérey—and this was the touch of genius of his conception—created the reserves out of nothing, by thinning out his line and reducing the number of troops on all the other sectors to the minimum indispensable for safety and sometimes going dangerously below it, so as to concentrate the largest possible number of men on the points where the decisive effort was to be made. In this way, although the total strength of the Allies was slightly below that of the enemy, he managed to have an overwhelming superiority at the points where he attacked.
The following were the forces of the Armée d’Orient. It comprised: 8 French divisions (30th, 57th, 76th, 122nd and 156th Metropolitan Divisions, and 11th, 16th and 17th Colonial Divisions), one cavalry group (1st and 4th Chasseurs d’Afrique and Morocco Spahis), some units not forming part of any division (2nd bis Zouaves, 58th Battalion of Chasseurs à pied, various battalions of Senegalese, Algerians, Annamites, etc.); one Italian division equal to rather more than two French divisions, and including two squadrons of cavalry and other units; 4 British divisions (22nd, 26th, 27th and 28th), each with some cavalry detachments, and the 228th Garrison Brigade; 6 Serbian infantry divisions (Shumadia, Danube, Morava, Yugoslav, Drina and Timok) and one cavalry division; 10 Greek divisions (Archipelago, Crete and Serres of the National Defence Army, 1st, 2nd, 3rd, 4th, 9th, 13th and 14th of the regular army), of whom one—the 9th—was not trained and was never sent to the front throughout the operations, and some cavalry; finally a few Albanian detachments.[41] In all there were 29½ divisions, of which the British, Serbian, Greek and the 5 Metropolitan French divisions comprised 9 battalions each, the French Colonial ones 12 each, and the Italian 18. The total strength was as follows:
| Divisions | Battalions | Ration Strength | Rifles | |
|---|---|---|---|---|
| French | 8 | 87 | 200,000 | 45,000 |
| British | 4½ | 40 | 135,000 | 32,000 |
| Italians | 1 | 18 | 44,000 | 10,000 |
| Serbs | 6 | 57 | 95,000 | 30,000 |
| Greeks | 10 | 84 | 100,000 | 40,000 |
| Total | 29½ | 286 | 574,000 | 157,000 |
The Allied artillery amounted to 1,600 guns, the machine guns were 2,680, the aeroplanes 200.
The total number of enemy battalions was somewhat less than that of the Allies (282), but their effectives were stronger—they amounted to about 700,000 men, including 204,000 rifles, while the men who could be regarded as combatants were 400,000. Their artillery was weaker as regards the number of guns (1,300). Until a few months previously they had been far stronger in artillery, but during the summer the Germans had withdrawn many of their batteries from the Eastern front and sent them to France, whereas the Allied artillery had been steadily strengthened. The enemy, however, still retained a larger number of heavy and medium calibre guns than we had, as well as several guns of greater calibre and range than anything on our side. Their machine guns were 2,530, and their aeroplanes only 80.
In view of the Allies’ inferiority in numbers, an attack could be delivered only by concentrating large forces on one point, and leaving the other sectors to be held by weak forces or by Greek troops of uncertain value.
General Franchet d’Espérey began by forming a new Army Group, called the Central Franco-Serbian Group, concentrated in the Moglena area. He withdrew from the Serbian front the Danube and Shumadia divisions, leaving the other four, reinforced by two French divisions—the 122nd (General Topard) and the 17th Colonial (General Pruneau)—attached to the II Serbian Army, by 13 heavy batteries and 30 trench guns, besides several flights of aeroplanes, engineer detachments, etc. He had asked for and obtained fresh technical material from France. The whole group was commanded by Voivod Michich. The Commander-in-Chief had decided on this plan as early as the end of June, and had devoted the following two and a half months to the necessary preparations. New roads and décauvilles had to be constructed, heavy artillery transported to a height of 1,800 metres, new aviation camps laid out, certain units specially trained. The 17th Colonial and the 122nd Divisions did not take over their sectors until the beginning of September. Although all movements were carried out at night, the enemy became aware that something was in preparation. In order to deceive them as to the point where the attack was to be delivered, a number of raids were carried out on various sectors.