Complex sentence: A main concern for the United States was the cost of potentially raising the military standards of the new Eastern European members, which estimates put at $10 billion USD, which could lead to the treaty being rejected by the Republican-held U.S. Senate.
Split into simpler sentences: A main concern for the United States was the cost of potentially raising the military standards of the new Eastern European members. Estimates put this cost at as much as $10 billion USD, which participants worried could lead to the treaty recognizing the new members being rejected by the Republican-held U.S. Senate.