There was a war already at the time of the Declaration and the DOI was not a legal act of secession as you want to frame it but exactly what it is titled a ‘declaration of independence’. The DOI does not mention secession it declares Independence.
You and yours want to claim that the Constitution and the Founders agreed with your right to secession but there is nothing in the Constitution at all to back up your claim. The legality of unilateral secession is clear – the Constitution does not allow it. Revolution though is of course a natural right.
Better study up on the Founders. From the Ratification of the Constitution by the State of New York; July 26, 1788 [my bold below]:
WE the Delegates of the People of the State of New York, duly elected and Met in Convention, having maturely considered the Constitution for the United States of America, agreed to on the seventeenth day of September, in the year One thousand Seven hundred and Eighty seven, by the Convention then assembled at Philadelphia in the Common-wealth of Pennsylvania (a Copy whereof precedes these presents) and having also seriously and deliberately considered the present situation of the United States, Do declare and make known. ...
That the Powers of Government may be reassumed by the People, whensoever it shall become necessary to their Happiness; ...
... Under these impressions and declaring that the rights aforesaid cannot be abridged or violated, and that the Explanations aforesaid are consistent with the said Constitution ... We the said Delegates, in the Name and in the behalf of the People of the State of New York Do by these presents Assent to and Ratify the said Constitution.
Hamilton and future Chief Justice John Jay voted for the ratification document that contained those statements above.
The legality of unilateral secession is clear – the Constitution does not allow it.
Please show me where secession is outlawed in the Constitution.
Madison said the following on July 24, 1788 to the Virginia ratification convention [my bold and underline below]. Madison and future Chief Justice John Marshall [along with three other Federalists] wrote something similar into the Virginia ratification document and then voted along with a majority of the Virginia convention to pass that ratification document.
That resolution declares that the powers granted by the proposed Constitution are the gift of the people, and may be resumed by them when perverted to their oppression, and every power not granted thereby remains with the people, and at their will. It adds, likewise, that no right, of any denomination, can be cancelled, abridged, restrained, or modified, by the general government, or any of its officers, except in those instances in which power is given by the Constitution for these purposes. There cannot be a more positive and unequivocal declaration of the principle of the adoption — that every thing not granted is reserved. This is obviously and self-evidently the case, without the declaration.
Secession was not prohibited in the Constitution. The power to prohibit secession was not delegated to the federal government or to other states that might oppose the secession of a given state. Secession remained in the powers reserved to the states or the people.
The Thirteenth Amendment to the Constitution of the United States was proposed by the Thirty-Eighth United States Congress, on January 31, 1865. The amendment was adopted on December 6, 1865, when Georgia ratified the amendment. In a proclamation of Secretary of State William Henry Seward, dated December 18, 1865, it was declared to have been ratified by the legislatures of twenty-seven of the then thirty-six states. This begs the question of whether secession had actually occured?
In 1787 the Congress of the Confederation of the United States passed the Northwest Ordinance which established what is now Ohio, Indiana, Illinois, Michigan, Wisconsin, and Minnesota as Northwest Territory and established the population required for statehood as 60,000 people. The ordinance was affirmed again by Congress under the current US Constitution in 1789. In 1802 the Enabling Act was passed telling Ohio how to become a state. A delegate would be elected for every 1,200 to go to a state convention which would determine whether or not they wanted to be a state and if so to decide how delegates would be chosen to write a constitution. The constitution must be “republican, and not repugnant to the ordinance of the thirteenth of July, one thousand seven hundred and eighty-seven, between the original States and the people and States of the territory northwest of the river Ohio.” Congress would then vote on whether or not to accept the constitution and the statehood of Ohio.
Given that the rebel States never lost their pre-rebellion geographic boundaries, they being instrumental in ratifying the 13th Ammendment (abolishing slavery) prior to their ‘re-admission’, and Andrew Johnson - who maintained Lincoln’s lenient policy of reconciliation - issued a formal pardon for the entire South on 29 May 1865, it can be argued that The Reconstruction Acts of 1867 were merely instruments of political reconciliation.
To that end the Acts divided the Southern states (except Tennessee, which had been readmitted to the Union in 1866) into five military districts. Military provost marshals were appointed to administrate martial law over the peoples of the former Confederacy, whereby general civil order was enforced, i.e., citizen and property rights upheld through military tribunal and/or commissions so as to oversee the reorganization of State and local civil governments until such time that self-governance could be restored.
The Acts stipulated that civilian rule and full state rights would only be restored after the States had adopted constitutions based upon universal male suffrage and ratified the Fourteenth Amendment. By 1868 all but three states were readmitted under these conditions, Mississippi, Texas, and Virginia finally acquiescing in 1870. The Fifteenth Amendment, ratified in March 1870, aimed to guarantee black suffrage in the South. The new state governments in the South were usually Republican and governed by blacks, carpetbaggers, and scalawags (white Southerners who supported the Union during the Civil War).
Special consideration and status was, however, conferred to those regions and areas which were deemed having been continuously loyal to the Union during the conflict and were named by county in the Emancipation Proclamation: “Louisiana, (except the Parishes of St. Bernard, Plaquemines, Jefferson, St. John, St. Charles, St. James Ascension, Assumption, Terrebonne, Lafourche, St. Mary, St. Martin, and Orleans, including the City of New Orleans)... and Virginia, (except the forty-eight counties designated as West Virginia, and also the counties of Berkley, Accomac, Northampton, Elizabeth City, York, Princess Ann, and Norfolk, including the cities of Norfolk and Portsmouth[)], and which excepted parts, are for the present, left precisely as if this proclamation were not issued.” These are the Louisiana and Virginia counties exempted from the Proclamation because even though they practiced slavery, were still regarded as loyal to the Union.
As each of the Southern states were ‘redeemed’ one-by-one by conservative political groups and influence of the Republican Party dwindled below the Mason–Dixon Line - corruption and incompetence certainly being a significant aspect of this period - the end of Reconstruction marked the end of contemporary attempts towards giving all citizens social equality. The end of Reconstruction was official with the withdrawal of Federal troops from the formerly States-in-Rebellion by President Rutherford B Hayes in 1877.
That notwithstanding, however, many states continued to practice discrimination and segregation; Jim Crow laws ubequetious to every Southern state disenfranchised all black citizens (stripping them of the legal right to challenge or contest the very laws and codes that oppressed them); white supremacist groups such as the KKK ran amok (virtually unchallenged by regional and local authority for nearly 1/2 century). The lack of an independent economic base, moreover, meant that blacks in the South were not able to advance socio-economically in any significant manner for many decades after Reconstruction ended.