it doesn't say anything about it. You can infer that they have the right to do it from the declaration of independence but that too started a war.
No, you can infer the right to do it from the 10th Amendment in the U.S. Constituion that specifically address powers that aren't granted to the federal government, but aren't prohibited to the states....
It doesn't say shit about it in the 10th.
WTF? Read the 10th Amendment again. Now tell me how it doesn't apply to this issue considering "secession" is not a power granted to the federal government (In the Constitution) and it's not a power prohibited to the states. The 10th Amendment doesn't say jack crap about traffic laws, education, etc... Which allows states to control and regulate these types of things via powers reserved by the 10th Amendment.