tenses - By the [point in time] + past simple / past continuous / past perfect

I've come across two sentences in "My Grammar Lab Advanced" that I found really confusing and made me want to reevaulate my understanding of the expression "by the + point in time" used in conjuction with various past tenses. The sentences are the following:

By the middle of 2008 many parts of Europe experienced / were
experiencing / had experienced the worst economic recession for
decades

and the second one

By the third month of the war rebel forces took / were taking / had
taken most of the province

My understanding of the expression By the + point in time (e.g. By the fourth day of the struggle) is that it's quite similar to the expression By the end of + point in time (e.g. By the end of 2010) which, according to a vast majority of English grammar coursebooks should be complemented with the Past Perfect (e.g. By the end of 2010 we had managed to save a lot of money). My question is: are all the three tenses (the Past Simple, Past Continuous, Past Perfect) correct in both sentences? Is there any rule governing the proper use the expression By the + point in time with the three above-mentioned tenses?
I'm pretty lost here, any help would be much appreciated.