I tend to point to Machiavelli's words on alliances as having a broader meaning that can be seen in every day life. He makes a general case, in The Prince, that there are times when it is beneficial to make defensive allies, that is to stand together against an attack, or to avoid additional punishment, but,
"And here it is to be noted that a prince ought
to take care never to make an alliance with one more powerful than himself
for the purposes of attacking others, unless necessity compels him, as is
said above; because if he conquers you are at his discretion, and princes
ought to avoid as much as possible being at the discretion of any one."
Which at the level of class war I might put forth in this way: If we start from the idea that such things as privilege exist, and the related idea that members of privileged classes have a vested interest, of which they are not perfectly conscious, in maintaining their privileges, then it should be seen that such privileged individuals are incapable of being "good" allies. Their heightened privilege in combination with the subversive force of their class interest implies that they will tend toward one of two relationships with the underprivileged class to which they have declared alliance.
1) Their interest in maintaining their privilege, or more broadly the expression of their class interests, will disincline them from exerting themselves fully toward the elimination of their privileges, or their class. The consequence of this is ostensibly a liberalization of the radical political agenda in question, the "allies" guiding an agenda which cuts them out of power instead toward one which merely renegotiates, that is reforms, the relationship between they and those they oppress.
2) Alternatively, the ally from the more privileged class, that is the more powerful ally, may be inclined to, perhaps consciously, or perhaps through the accretion of many individual actions of benefit seeking, use the oppressed group to which they ally themselves as a platform to advance their own interests. An example of this would be white politicians who advance their careers fighting for racial equality, but plenty of others may be found. It could be further argued that even if they are advancing their own interests, nonetheless they aid the oppressed group in ending their own oppression, but even then, I think there is substantial evidence that this is stymied, in practice, by the tendency of such power seeking individuals to do so using well trod paths that again lead toward the liberalization mentioned in the first condition.
So, I would say that if one is to realize the end to their oppression, they ought to seek accomplices who share their interests and eschew all alliances.