The bracha on bris mila for conversion uses the phrase, "commanded us to". But we are not commanded to convert anyone who asks. So why does the bracha use that phrase?
The bracha for bris appears on pages 58 and 59 here
I will try to give you some elements of answer.
There are several opinions concerning the beracha before and after the Mila. The Beracha before the Mia: Two girsaot in Gemara. Acccording to our Gemara (Shass Vilna) in Shabbat 137b the beracha is "al hamila" as for a Jewish boy. The girsa of Rif and Rambam taught that the beracha is "to circumcise the converts..." ("למול את הגרים"). The halacha followed this last version. This beracha is referring to the fact that the custom is to make the mila before the mikveh, Bet Yosef explained that to make the mila before the mikve is not really a necessity. But he reported a Magid (Isure Bia 4) in name of Ramban to explain what is the rational for circumcision first. Circumcision is less easy than mikve in the conversion process. There is a risk that the man would become discouraged before the mila. We prefer to avoid such cases after mikve. Because a Jewish man who doesn't want to make mila is a bad result.
Consequently, the mila is generally made before the mikve. In this case the mohel cannot bless "al hamila", "on fulfilling of mitsvat mila" because the man is still not commanded. Kesef Mishne (Mila 3, 4) explains that for this reason Chachamim made an other Beracha which point not on the present circumcision, which is not really a mitsva. The mitsva is that this man will be actually mahul when he would become Jewish. This beracha, is a kind of birkat hashevach, a praise of G-d, not a blessing for fulfilling a commandment (Kesef Mishne Mila 3, 4) . Rambam himself said (in the end of chapter 3 of hilchot Mila, paraphrasing AZ 26b) that to circumcise a non Jewish man who wants to convert is allowed, (not a mitsva but is allowed) .
So, in summary the nusach "to circumcise converts" is for a blessing of praise, not for mitsva fulfilling.
All poskim mentioned the fact that to bless it before the immersion is not adapted because the still non Jewish candidate has no duty to immerse. On the other hand Gemara Pesachim 7b cited this case as an exception to the principle following which a blessing on a mitsva needs to be said before the mitsva making, following the understanding of Rabenu Chanan'el and the Rif. The immersion in the mikve has a double effect, to make the man Jewish, and to clean him from a special uncleanness. For a Jewish man, to rid of this uncleanness is a mitsva. He says this beracha as already member of Jewish nation, as a beracha acharona, regarding the cleaning of the uncleanness.
The wording of the blessing is "to circumcise the converts". That means that once someone qualifies to be converted, there is a mitzvah (see Krisus 9a where the Talmud learns from the verse in Bamidbar 15:15 that a convert must undergo milah) to circumcise them. There is no implication that we are supposed to circumcise just anybody!