“They” say that Stalin believed Germany was not ready for war yet and would only attack the next year, so he didn’t want to tip his hand by visibly preparing for war too early.

In a way he was right on the first point - Germany wasn’t ready for a protracted winter-time conflict. However they attacked anyway, brazenly confident they would be finished before the winter cold sets in, and also maybe desperate to strike before Stalin has a chance to strike first.

Nothing to do with friends and betrayals, just a series of miscalculations on all sides.

Or so “they” say.