re: If secession was legal then what right did the North have to keep the South in the USA?

Let me give you the real answer. 1) FIRST, the war was absolutely fought over slavery. Read South Carolina "Reasons for secession", they are almost exclusively regarding slavery. They fought for states' rights, but states' rights to own and manage slaves.

prior to the civil war, each state signed a pact with the Federal Government. The federal end of it stated it must uphold state laws and other stuff. South Carolina, the first state to secede, that the US government broke the pact by not forcing other non slave holding states (the north) to return runaway slaves. The US government claimed it couldn't uphold one states laws that broke another's. The us government also stated that it was constitutional to use military power to end rebellions.