The context for this sentence is the early part of the American Civil War. Virginia is the American state, which has declared independence from the United States. The Fugitive Act is the Fugitive Slave Act, which requires slaves to be returned to their owners even if they are in a part of the United States where slavery is illegal. More background information on the sentence can be found here: https://www.archives.gov/publications/prologue/2017/winter/summer-of-1862 (fifth paragraph: A lawyer before the war, Butler concluded that Mallory’s slaves were “contrabands of war” and could be taken from the enemy. Butler told Carey “that the fugitive slave act did not affect a foreign country, which Virginia claimed to be, and she must reckon it one of the infelicities of her position that in so far at least she was taken at her word.”)
So, with that background information at hand, we can actually try and understand the sentence. One way to do this is to expand it into several simpler sentences:
The Fugitive Slave Act does not apply to foreign (to the US) countries. Virginia claims to be (part of) a foreign country. Virginia must accept that it is unlucky for Virginia to be a foreign country in this instance, assuming that we accept Virginia's word that she is a foreign country.
Notably, the speaker doesn't accept that Virginia is in fact a foreign country, but knows that the person he is talking to (Major Carey, a member of the Confederate army speaking on behalf of Colonel Mallory) is quite insistent that it is. Thus, the speaker is trying to force a choice-either Carey and Mallory admit that Virginia is part of the United States so that the Fugitive Act applies (and they therefore stop fighting for the Confederacy), or they accept that they are not getting their slaves back by referencing American law.
m afraid I cannot ,because the sentence here is just a randomly given example from "English dictionary-offline",which dosen
t even tell more than a simple sentence without any title or source of original quotation, and I just came across this one a little bit perplexing when I searched word "insofar",thus I came to ask for that.