A 1972 agreement between Canada and the United States reduced

Dear GMATNinja

I had the same query! It seems strange to me that the sentence does not use present perfect (because the reduction began in the past and is continuing into the present) or at least simple present tense for the "reducing". Because from my understanding the agreement still seems to be in place and therefore still reduces the amounts of phosphates. (Especially since the dumping is also in the present tense and therefore it seems reasonable to assume that the phosphate dumping is still an ongoing issue and the treaty is still curtailing the amounts of phosphates that can be dumped)

I like your explanation even though I think it is not quite comparable (at least by the weird logic in my head hahah). The treaty described in the sentence describes some form of limit. If the treaty were to be abandoned there would be no limit and therefore (most likely) no reduction. In other words, the base case is that there is no treaty, therefore only if some kind of agreement is upkept is there any reduction and therefore the agreement continuously reduces the amount of phosphates.

HOWEVER, in your example of an act which ended a war, I believe the base case is that there is no war. In other words, the normal state is that there is no war and only in an exceptional crisis is there a war between two countries. Therefore, the ending of a war is a momentary event which restores the normal state (of peace) and could occur in the past. It does not have to be in present because the base case is that there is no war. Therefore even if there would no peace treaty between two countries there would be no war and therefore the peace agreement does not continuously prevent war. On the contrary only when another war would be declared would another war begin. This seems very different from the case of the curtailment of phosphate pollution because this agreement continuously reduces the phosphate pollution.

Thank you, kudos to you!