https://en.wikipedia.org/wiki/Sovereign_immunity_in_the_Unit...
> In the United States, the federal government has sovereign immunity and may not be sued unless it has waived its immunity or consented to suit.[7] The United States as a sovereign is immune from suit unless it unequivocally consents to being sued.[8] The United States Supreme Court in Price v. United States observed: "It is an axiom of our jurisprudence. The government is not liable to suit unless it consents thereto, and its liability in suit cannot be extended beyond the plain language of the statute authorizing it."[9]
The only reason the United States can legally appear as a defendant in a civil suit currently is because Congress has consented to being sued:
> The United States has waived sovereign immunity to a limited extent, mainly through the Federal Tort Claims Act, which waives the immunity if a tortious act of a federal employee causes damage, and the Tucker Act, which waives the immunity over claims arising out of contracts to which the federal government is a party. The Federal Tort Claims Act and the Tucker Act are not the broad waivers of sovereign immunity they might appear to be, as there are a number of statutory exceptions and judicially fashioned limiting doctrines applicable to both. Title 28 U.S.C. ยง 1331 confers federal question jurisdiction on district courts, but this statute has been held not to be a blanket waiver of sovereign immunity on the part of the federal government.
But in general, the government of the United states is immune from civil suits, and Congress can decide at any time -- even during an active case -- to decide to not want to be sued anymore.
The governments of the states -- being the ultimate sovereign authority of the United States of America -- are also generally immune from civil suit, except in limited circumstances. I believe most have however consented to being sued.
On this matter, the supreme court has said:
> we have understood the Eleventh Amendment to stand not so much for what it says, but for the presupposition of our constitutional structure which it confirms: that the States entered the federal system with their sovereignty intact; that the judicial authority in Article III is limited by this sovereignty, and that a State will therefore not be subject to suit in federal court unless it has consented to suit, either expressly or in the "plan of the convention." States may consent to suit, and therefore waive their Eleventh Amendment immunity by removing a case from state court to federal court.