Colonel Anderson they had met in Berlin just after the Kaiser had declared war upon France.  The colonel, lieutenant then, and Major Derevaux, a Frenchman, had taken the boys with them on their flight and the four had later encountered each other in many strange and unexpected places.

Stubbs they also had met while on one of their many missions and had earned the little man’s undying gratitude; but he had repaid whatever they had done for him, with interest, more than once.

The boys, in their latest exploit, had been with the Italian army in the Alps.  Two of the four friends having fallen into the hands of the enemy, the others had entered the enemy’s lines in an effort to effect their escape.

It was a daring adventure, but after a fight and chase, the four had managed to seize the airship in which we now find them and had at last fought their way clear.  They had then held a council of war and decided that it was best to head for the Balkans, rather than to run the gauntlet of the Austrian flying craft which kept constant vigil in the direction of the Italian lines.

Hal and Chester, typical American lads, were large and strong for their ages, which were within a year of each other, seventeen and eighteen now.  In the rough lumber camps of the north, the two had had considerable experience in the use of firearms and the art of self-defense—­fists.  Also, during the school term each had practiced the use of the sword until, though by no means experts, they could give a fair account of themselves with this weapon—­as each had done more than once.

Fortunately, both lads had made a study of languages and spoke French and German fluently.  They never had trouble on that score.

The great war up to this point had not gone as successfully as the Entente Allies had hoped in the early days.  The German lines on all fronts were seemingly stronger than ever before.  Even the entrance of Italy into the war on the side of the Allies had failed to turn the balance, as it had been confidently expected it would.  East and west, the German lines held, while in the Balkans the enemy was even now advancing against the heroic little Serbian army, which, before many days, was to be forced to relinquish its country to the iron heel of the invader.  Montenegro, the smallest factor in the war, still was fighting hard—­the rugged and gigantic mountaineers giving a good account of themselves upon all sides.

This was the situation, then, as the airship containing Colonel Anderson, British officer, Anthony Stubbs, American war correspondent, and Hal Paine and Chester Crawford sped southward over Austria.

Several hours after the sinking of the Austrian aeroplane Chester spoke.

“Where do you suppose we are now, Hal?” he asked.

“I believe we must have crossed the frontier,” replied Hal.  “However, we’ll wait another half hour before descending to have a look.”