〜ばこそ
precisely because (a good motive)
Verb ば-form + こそ (〜ていればこそ, 〜であればこそ)
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ばこそ means "it's precisely because…", and stresses a positive, often misunderstood motive: 子どものためを思えばこそ、厳しく叱るのだ, "it's precisely because I care about my child that I'm strict with him".
The second half often ends in のだ or のである, explaining why someone acted as they did. The motive is usually love, trust, expectation or health, something good behind a harsh-looking action.
It's a literary form of からこそ (N3's こそ). Watch the shape: からこそ follows the plain form (思うからこそ), while ばこそ follows the ば-form (思えばこそ). It's common as 〜ていればこそ and 〜であればこそ.
In classical Japanese, ばこそ could also mean "far from", but that use is rare today.
Examples
子どものためを思えばこそ、厳しく叱るのだ。
It's precisely because I care about my child that I'm strict with him.
君を信頼していればこそ、この仕事を任せるのだ。
It's precisely because I trust you that I'm leaving this job to you.
健康であればこそ、好きなことができる。
It's only because you're healthy that you can do what you love.
愛していればこそ、別れを選んだのだ。
It was precisely because I loved her that I chose to leave.
期待していればこそ、厳しいことを言うのだ。
I say harsh things precisely because I expect a lot of you.