Question

complete the sentence with the best relative pronoun. at the bloody battle of antietam in 1862, there were nearly ten thousand casualties among the confederate soldiers, the union army forced to retreat to virginia.

Explanation:

The relative pronoun "which" is used to refer to the previous clause (the situation of nearly ten thousand casualties among Confederate soldiers). It is used in non - restrictive relative clauses (clauses that add extra information and are separated by commas). Here, the clause "which the Union army forced to retreat to Virginia" is giving more information about the situation described in the first part of the sentence.

Answer:

which