Snowball+was+in+league+with+Jones+from+the+very+start!+He+was+Jones's+secret+agent+all+the+time.

By saying this, the Napoleon team is trying to cause a terror among the animals in Animal Farm which will make they rely on them.

Q Park: Using his current power, Squealer and Napoleon is trying to eliminate all of his enemies. Within the animal farm, no one else opposes them, but they know that snowball surely would be a threat if he comes back, whether with the farmer or not. Therefore, I thought that this quote was used to show the misuse of power to maintain dictatorship.