If I am correct in the Reconstruction 11 CSA states were readmitted into the USA. How come? If the argument of the USA was that states did not have the right to secede, then their original secession had been legally invalid - it did not legally happen. If so why did these 11 states needed to be "readmitted" into something they had supposedly never left de jure? Does this readmission bussines therefore amount to a recognition of the USA that 11 states had indeed legally seceded and that what it was engaged in 1861-1865 was an invasion of the CSA?
If you look up inconsistent in the dictionary, you'll see "Civil War" as a synonym.
Does this readmission bussines therefore amount to a recognition of the USA that 11 states had indeed legally seceded and that what it was engaged in 1861-1865 was an invasion of the CSA?
yes.
http://en.wikipedia.org/wiki/Virginia_v._West_Virginia
Of possible interest is also the case of West Virginia's secession from Virginia. West Virginia shouldn't have been recognized since it consisted of an existing state's territory but said state didn't give consent to have it secede. The feds recognized it anyway.
Secession is completely legal when it benefits the powers that be.
The issue is extremely complicated.
Abraham Lincoln (backed by the Moderate Republicans) proposed the so called Ten Percent Plan, according to which any CSA State could be readmitted in the Union when 10% of the voters (according to the 1860 census, hence excluding freed slaves) had taken an oath of allegiance to the USA. Voters would then elect delegates to draft new State constituttions and form new legislatures. The Plan also included a complete pardon for all bar the highest ranking CSA military officers and government officials.
Radical Republicans thought this plan too lenient and in 1864 they attempted to pass through the much more stringent Wade-Davis Bill. While it passed through the Senate, Lincoln vetoed it.
After Lincoln was assassinated in 1865, Andrew Johnson from Tennessee became president. He had an even more lenient stance than Lincoln towards the Reconstruction: his intentions were to nominate provisional governors for those States (like Virginia and Texas) which hadn't embraced the Ten Percent Plan who would oversee elections in 1866 to draft new constitutions as soon as possible, oath of alleginace or not. His stance on pardons was the same as Lincoln's.
Johnson clashed continously with the Radical Republicans, who in 1866 won a landslide election and even attempted to impeach Johnson to avoid being vetoed continously.
Following the Reconstruction Act in 1867 and the election of Ulysses Grant (a Radical Republican) in 1868,the South descended into the excesses commonly associated with the Reconstruction.
This was a relatively short lived period: by 1872 Grant had alienated the bulk of the Republican party because of the corruption in his administration and the continous use of Federal troops to prop up Radical Republican regimes in the South. The Panic of 1873 and the outbreak of organized armed resistance to carpetbagger regimes, chiefly in Lousiana, Mississippi and the Carolinas, signalled the end of the Reconstruction Era.
Conventionally this period ended when Rutherford Hayes, Grant's successor, removed Federal troops from New Orleans and other major Southern cities in 1877.
So you see: it was never about rule of the law. Even Lincoln contented himself with a token oath of allegiance, and Johnson with much less. The hullaballoo was created by rabid Radical Republicans for political and financial reasons. Nothing new under the sun.