"Such is not the plan of the general staff. Their reasons I cannot tell you; but at a guess I should say it is because they hope that, by a juncture with the Anglo-French forces, they may hope to show an effective front until Italy can throw an army to their support, or possibly until the long expected Russian offensive materializes."

"Then we shall have to bring some pressure to bear upon Greece," said the younger man. "We cannot permit that. Bulgaria must get in the game sooner and thus foil such a plan."

"Well, you probably know best," said the officer, "but remember one thing. To all intents and purposes, Bulgaria is still neutral.

Announcement that she has decided to cast her lot with the Central Powers, if premature, undoubtedly would spoil many plans. Particularly, if it came to the ears of the Anglo-French commander at Saloniki."

"Exactly," replied the young man. "Our plans now are to permit the Allies to advance a considerable distance toward Belgrade, and then to have Bulgaria declare war at the psychological moment."

"A good plan, that," returned the officer. "But I must get back now. My absence will be noticed and I do not care to arouse suspicion."

The men moved off.

Chester and Colonel Anderson gazed at each other.

"Rather neat little play," said Colonel Anderson.

"Rather," repeated Chester dryly.

"And to think," continued Colonel Anderson, "how leniently Bulgaria has been treated by the Allies. Well, her day of reckoning will come."

"We'll have to get word of this to the Serbian commander in Belgrade,"

said Chester.

"So we will," said the colonel. "And also to the commander of the Anglo-French forces in Saloniki."

"Let's get back then and see if Hal has the machine fixed so she'll fly."

They retraced their footsteps; and even as they arrived, Hal arose from his position above the aeroplane.

"She'll go now all right," he said. "All aboard!"

Stubbs, Nikol and Ivan hesitated and Stubbs protested. Chester drew Hal aside for a moment and told him what he and the colonel had learned. Hal wasted no further time.

"In here with all of you," he commanded gruffly. "We're going right now."

The others hesitated no longer, and a few moments later the big machine was flying swiftly toward the Serbian capital.

CHAPTER XX.

THE END OF A TRAITOR.

It was two years after the outbreak of the great war that the Austro-German armies were hurled forward in a great and final effort to crush Serbia. Since the early days of the struggle, heavy battles had been fought upon the Austro-Serbian frontier, with success first to one side and then to the other.

Belgrade, the Serbian capital, had been bombarded time after time by the great Austrian guns and once the city had been occupied by the foe.

Later, however, the Serbians had driven out the invader and reoccupied the capital. And now, the Austrian army, reinforced by a hundred thousand Germans, bringing the total number of troops to half a million, was again knocking at the gates of Belgrade; and the Serbians, realizing the utter hopelessness of their cause unless aid arrived from the Anglo-French troops at Saloniki, were preparing to flee.

This was the situation when the aeroplane bearing Hal, Chester and their friends descended just outside the city.

Hardly had they alighted when they were taken in charge by a squad of Serbian troops. Colonel Anderson, acting as spokesman for the party, explained their presence in a few well-chosen words and asked to be taken to the commanding officer. There was considerable red tape to go through before the friends finally were ushered into the presence of the Serbian commander, and that worthy immediately informed them he had but a few moments to give them.

Colonel Anderson, therefore, came to the point at once. He told him of the conversation he and Chester had overheard a short time before.

"And you say one of the men wore a Serbian uniform?" asked the general.

"Yes, sir."

"You don't know who he is--you didn't hear his name mentioned?"

"No, sir; but I would know him again if I saw him."

"Good. You shall have the chance. Now, how far from the city do you say this conversation took place?"

"Must have been all of ten miles, sir."

"Then the men have hardly returned to the city yet. And you say you did not hear the name of the member of the general staff, the first traitor, or spy mentioned as having divulged information?"

"No, sir."

"Very well. Now I will leave all of you here for an hour or so. I have some matters to attend to. When I come back we'll see if you can identify the man you speak of."

The general bowed to them and took his departure, leaving them alone in his quarters.

From without a heavy cannonading could be heard.

"I guess the last advance has begun," said Chester slowly.

"You probably are right," agreed Hal. "And I feel sorry for these Serbians. If the British and French could only get here in time."

"Well, I don't see why they don't," declared Chester. "England has promised more than once since the war began that she would not permit Serbia to be crushed. Seems to me she should have taken some decisive action before now."

"You forget," said Colonel Anderson, "that England has her hands full in other parts of the great war theater--France, Belgium, the Dardanelles, Egypt, India and Africa."

"That's the trouble," said Hal. "England has too many irons in the fire.

That's where the Germans and Austrians have the edge, as we say in the United States. Their armies are not scattered all over the world."

"That's true enough," replied Colonel Anderson, "and it is, without doubt, the reason the Central Powers have not been crushed long ago."

Ivan now took a hand in the conversation.

"These wonderful tales you told me of my brother Alexis," he began.

"Well, what of them?" asked Hal.

"Why," said Ivan. "When I came with you I thought I should see some fighting. All I have done is fly through the air, like a bird, and hear a thousand miles of talk. I want to see some fighting, like Alexis saw."