The forgotten Corwin Amendment.

In a last ditch effort to prevent the Civil War, Congress proposed the Corwin Amendment:

“The Corwin Amendment was a proposed amendment to the United States Constitution passed by the United States Congress on March 2, 1861. Ohio Representative Thomas Corwin offered the amendment during the closing days of the Second Session of the 36th Congress in the form of House (Joint) Resolution No. 80. The proposed amendment would have forbidden attempts to subsequently amend the Constitution to empower the Congress to "abolish or interfere" with the "domestic institutions" of the states, including "persons held to labor or service" (a reference to slavery).
Corwin's resolution emerged as the House of Representatives's version of an earlier, identical proposal in the Senate offered by Senator William H. Seward of New York. However, the newly formed Confederate States of America was totally committed to independence, and so it ignored the proposed Corwin Amendment.
This proposed amendment is technically still before the states for ratification, because it was submitted to the states without a time limit. Since the Thirteenth Amendment (which abolished slavery) was adopted, the Corwin Amendment has had no realistic chance of being adopted.
Text
No amendment shall be made to the Constitution which will authorize or give to Congress the power to abolish or interfere, within any State, with the domestic institutions thereof, including that of persons held to labor or service by the laws of said State.[1]
Proposal by the Congress
On February 28, 1861, the House of Representatives approved the resolution by a vote of 133–65.[2] On March 2, the United States Senate also adopted it, 24–12.[3] Since proposed constitutional amendments require a two-thirds majority, 132 votes were required in the House and 24 in the Senate. As seven slave states had already decided to secede from the Union, those states chose not to vote on the Corwin Amendment.
Outgoing President James Buchanan endorsed the Corwin Amendment by taking the unusual step of signing it. Abraham Lincoln, in his first inaugural address, supported the Corwin Amendment: "[H]olding such a provision to now be implied Constitutional law, I have no objection to its being made express and irrevocable."[4][5] Just weeks prior to the outbreak of the Civil War, Lincoln penned a letter to each governor asking for them to support the Corwin Amendment.[6] However, Presidents play no formal role in the amendment process.[7]
Ratifications
Pursuant to Article V of the Constitution, consideration of the Corwin Amendment then shifted to the state legislatures. On May 13, 1861, the Ohio General Assembly became the first to ratify the amendment. The next to ratify was the Maryland General Assembly in January 1862. Later that year, Illinois lawmakers approved the amendment while they were sitting in session as a state constitutional convention rather than as a legislature, thus causing some to see this particular ratification as possibly invalid.[4]
Technically, the Corwin Amendment is still pending.”
Excerpts from:http://en.wikipedia.org/wiki/Corwin_amendment

It is not clear if the amendment would have covered territories. Arguably not. The federal government controlled territories. However, arguably a territory could have voted to become a slave state, and then slavery would have in that new state would have been protected by the amendment. Arguably, too many in the South were already committed to secession, and war if necessary. This amendment was arguably too little and too late. Perhaps an amendment similar to the one proposed in the post below might have worked if proposed early enough.