Answer:
But.
Step-by-step explanation:
Transition words are those words or phrases that show the relation of two sentences with each other. These words form a cohesion of the two related sentences.
In the given sentence, the word "but" acts as the cohesion for the two parts of the sentence "An uneasy peace prevailed between the Russian and the other allies" and "each other side was suspicious of the other. This word "but" shows the connection between the two separate sentences and how related they are, making it easy for the readers to understand the whole sentence. Thus, the transition word is "BUT".