Will the United States be brought to its knees?
Mrs. White made no such statement. In volume 1 of the Testimonies, she wrote, “This nation will yet be humbled into the dust,” but the setting makes it clear that she was referring to the Civil War (259).
As for end times, the closest thing I know to what you are seeking is a reference to the United States’ “temporal prosperity” needing restoration. Here it is, from The Great Controversy, page 590:
And then the great deceiver will persuade men that those who serve God are causing these evils. The class that have provoked the displeasure of Heaven will charge all their troubles upon those whose obedience to God’s commandments is a perpetual reproof to transgressors. It will be declared that men are offending God by the violation of the Sunday sabbath; that this sin has brought calamities which will not cease until Sunday observance shall be strictly enforced; and that those who present the claims of the fourth commandment, thus destroying reverence for Sunday, are troublers of the people, preventing their restoration to divine favor and temporal prosperity. Thus the accusation urged of old against the servant of God will be repeated and upon grounds equally well established: “And it came to pass, when Ahab saw Elijah, that Ahab said unto him, Art thou he that troubleth Israel? And he answered, I have not troubled Israel; but thou, and thy father’s house, in that ye have forsaken the commandments of the Lord, and thou hast followed Baalim.” 1 Kings 18:17, 18. As the wrath of the people shall be excited by false charges, they will pursue a course toward God’s ambassadors very similar to that which apostate Israel pursued toward Elijah.