"情况既然这样" can be translated into English as "Since the situation is like this" or "Given that the situation is as it is".
Both expressions convey the idea of acknowledging the current state of affairs and proceeding from there. The first one is more straightforward and commonly used, while the second one adds a touch of formality or emphasis on the given nature of the situation.
For example:
"Since the situation is like this, we need to come up with a new plan."
"Given that the situation is as it is, we should reconsider our approach."