N1
それまでだ
If … then it's all over, It's all in vain, That's the end of it; (と言えばそれまでだが) If you put it that way, there's nothing more to say
やっとここまでたどり着いた。ここでミスを犯せばそれまでだ。
We've finally made it this far. If we make a mistake here, it's all over.
Explanation
「それまでだ」 has 2 uses
1If … happens, it's all over
〜たら／〜ばそれまでだ: if X happens, everything is lost or wasted and nothing more can be done.
Ａ：「毎日毎日勉強していてとても疲れてるんだ。大学を中退したい…。」Ｂ：「でも、もう３年生だよ！あきらめてしまったらそれまでだよ。」
A: 'I'm studying every single day, I am so tired. I want to drop out of college...' B: 'But you're already in your third year! If you give up then it's all in vain.'
やっとここまでたどり着いた。ここでミスを犯せばそれまでだ。
We've finally made it this far. If we make a mistake here, it's all over.
2If you put it that way, there's nothing more to say
〜と言えば（言われれば）それまでだが: the speaker grants a simple explanation ('if you put it that way, there's nothing more to say'), then adds a reservation.
この内定を受諾することに興味がないだけと言えばそれまでだが、もういちどよく考えて決めたほうがいいだろうな。こんなチャンスは二度とないよ。
If you say you're simply not interested in accepting this job offer, there's nothing more to say, but I think you'd better think it over once more before deciding. You won't get a chance like this again.
彼女が日本語能力試験に一回で合格したのは彼女が才能に溢れているからと言ってしまえばそれまでだが、実は彼女も努力をしたんだよ。
If you say that she passed the JLPT on the first try because she is overflowing with talent then that is the end of the discussion, but the truth is that she also put forth an effort.