Context: In practice, because of sovereignty, any state can withdraw from any treaty at any time. The question of whether this is permitted is really a question of how other states will react to the withdrawal; for instance, another state might impose sanctions or go to war over a treaty violation.

Question: What factor, in practice, allows a state to withdraw from any treaty at any time?
Answer: sovereignty

Question: What really determines whether withdrawal from a treaty is permitted?
Answer: how other states will react

Question: What might a state do in response to another state's withdrawal from a treaty?
Answer: impose sanctions or go to war

Question: Although withdrawal from a treaty may not be possible in theory, when might it be possible anyway?
Answer: In practice

Question: Because of sovereignty when may a state withdrawal from a treaty?
Answer: at any time