いったん〜ば／たら

once (X happens), (it is hard to undo / Y inevitably follows)

Formation: いったん + V-ば / V-たら / V-と (often passive or potential)

いったん with a conditional means "once X has happened". The main clause then states a result that follows inevitably or cannot be reversed (取り返しがつかない, 止まらない, 元には戻らない). It is common in warnings and arguments. On its own, いったん can also mean "for the moment, temporarily" (いったん家に帰る), so the conditional is the clue. It is close to 〜たら最後, but 〜たら最後 is more emphatic and more colloquial.

Variants: いったん〜たら、いったん〜と、一旦〜ば

Examples

いったん信用を失えば、取り戻すのは難しい。
Once you lose trust, it is hard to win it back.
いったん話し始めたら、彼は止まらない。
Once he starts talking, he won't stop.

In real texts 1 occurrence(s)

遅れる生物多様性保全　地球の危機直視し行動を · 2026年11月5日

いったん破壊されれば取り返しがつかない。

Once they are destroyed, the damage cannot be undone.

いったん plus the conditional 破壊されれば (passive 破壊される + ば) means "once (ecosystems) are destroyed". The main clause 取り返しがつかない states the irreversible consequence. The pattern stresses that a single event creates a permanent new state, which is the core of the writer's argument for urgency. Do not read いったん here as "temporarily" (いったん休憩する "take a short break"). Followed by a conditional, it means "once".