〜までだ
Formation
Meaning
Expresses that since there are no other options, one will simply take the only remaining course of action. When used with これ or それ, it denotes that a situation has reached its absolute end/limit.
Note
A formal and decisive expression. It is often preceded by a conditional clause (〜たら、〜ば), suggesting that if a certain boundary is crossed, the speaker is resolved to take a simple, final action. It carries a nuance of resignation ('nothing else to be done') or firm resolve ('I'm prepared to do just that'). The variant '〜までのことだ' is even more formal. When used as 'これ・それまでだ', it means 'that's the end of it' or 'it's all over'.
Examples
もし彼が協力してくれないなら、一人でやるまでだ。
If he won't cooperate with us, I'll just have to do it alone.
電車が止まってしまったら、歩いて帰るまでだ。
If the train stops, I'll just have to walk home.
資金が足りなければ、銀行から借りるまでのことだ。
If we are short of funds, we will just have to borrow from the bank.
一度断られたからといって諦めない、もう一度頼むまでのことだ。
Just because I was refused once does not mean I will give up; I will just ask again.
万策尽きた、もうこれまでだ。
I've run out of options. This is the end.
信頼を失ってしまっては、それまでだ。
Once you lose trust, it's all over.