Grounds noun – Your basis for belief or disbelief; knowledge on which to base belief. Occasion is a synonym for grounds in basis topic. In some cases you can use "Occasion" instead a noun "Grounds".
On the one occasion where it's offered by the superior force, the inferior force refuses on the grounds that they can't trust the superior force to honor the agreement.
On one occasion Cole actually vetoed a perfectly good plan to discover the identity of Jade's online romance on the grounds that it wasn't interestingly wacky enough.