Sentence Correction

Hi , this is one of the sentences that i came across. Can anyone help me out with it.
"Where once the union had acquiesced to the prejudices of its English speaking members by supporting the imposition of an alien tax on immigrants workers, after 1987 the united mine workers made a determined effort to enlist Italians and slaves In its ranks"

The first word. Should it be where or while?

I personally thought it should be while and many have supported me in the gmat forum though they have marked the correct one as where

rahulramteke wrote:Hi , this is one of the sentences that i came across. Can anyone help me out with it.
"Where once the union had acquiesced to the prejudices of its English speaking members by supporting the imposition of an alien tax on immigrants workers, after 1987 the united mine workers made a determined effort to enlist Italians and slaves In its ranks"

The first word. Should it be where or while?

I personally thought it should be while and many have supported me in the gmat forum though they have marked the correct one as where

I feel usage of while is wrong in the above sentence because with while two events should be happening at the same time but if we see the above sentence then once the union had acquiesced to the prejudices of its English speaking members by supporting the imposition of an alien tax on immigrants workers & after 1987 the united mine workers made a determined effort to enlist Italians and slaves In its ranks are not happening at the same time and are also mutually exclusive events...........

I think "While" will be correct. I think "while" can also be used to show a contrast.

I am not sure if "where" is correct. May be it is...Did you find this sentence in a trusted source. It sure looks like a good one except for the "where".

thanks guys . Any expert comments on this please.