Poirot tells M. Bouc, a fellow Belgian, that a “little affair in Syria” brought him to Syria. What precisely Poirot was doing is not spelled out in words of one syllable, but as good mystery reading sleuths, we can use the clues provided to determine why Poirot was in Aleppo.

Lieutenant Dubosc, for example, overhears a bit about what went on:

The General’s—his General’s—temper had grown worse and worse. And then there had come this Belgian stranger—all the way from England, it seemed. There had been a week—a week of curious tensity. And then certain things had happened. A very distinguished officer had committed suicide, another had suddenly resigned, anxious faces had suddenly lost their anxiety, certain military precautions were relaxed. And the General, Lieutenant Dubosc’s own particular General, had suddenly looked ten years younger.

Lieutenant Dubosc also overhears the General thanking Poirot, saying:

You have saved the honour of the French Army—you have averted much bloodshed! How can I thank you for acceding to my request?

What we can deduce is that the General becomes aware that a mutiny, or perhaps army coup d'etat, is being planned in the Syrian armed forces. (We remember that at this time Syria is a French colony so Syrian nationals may have been plotting against France.) The general is very nervous and upset, but unable to unravel the mystery of who is behind this potentially treasonous activity. Therefore, he calls in Poirot, who gets to the bottom of it and exposes the traitors. One commits suicide and another resigns from the army. Poirot has averted the threat, and the general is very grateful to him. Poirot then reminds the general that he once saved Poirot's life.

From this, we can also deduce that Poirot is at the top of his form at he sets out to return home on the Orient Express.