The "Government" has not promised anything. The government is bound to respect the Constitution, not make ‘promises concerning basic civil liberties’ It is not about promises like tax cuts, a new road, or whatever. It is law and we are a nation of laws, not promises.
Further, when we talk of 'Government" in the Federal sense, were must remember the Separation of Powers that the Framers build into the Constitution to help protect our liberties. Congress can pass a law that is unconstitutional, such as they did last month on CFR. They have often passed laws that ‘revoke’ civil rights. The executive can refuse to enforce that law, or the judiciary can overturn it. The executive branch can exceed their constitutional authority in enforcing laws. Congress can pass new laws or cut off funds to the executive in response. And the Judicial branch to rule on the constitutionality of both the laws congress passes and the actions of the executive in enforcing laws. Congress and the Executive very often get whacked by the Judicial, but by the same token, the Judicial can overstep its authority which then can be circumvented by new laws from Congress. It is a dynamic tension between branches that has served us remarkably well. When serious conflicts arise such as Roe v Wade, it is up to us as a people to address then in a political setting.
From your statement, however, you seem to imply that the state governments are more trustworthy than the Federal government when it comes to civil liberties. Historically, the opposite is true. The most common civil liberty abuses in our history have been committed by state governments, not the federal. The Judicial can and often does strike down state laws on Constitutional grounds which is where most of our serious constitutional disputes arise be it Dred Scott, Roe. V Wade or Brown v Board of Education.
If this is the case, a state elected civil government can vote to leave and that is moral.
If the situation arises where the Legislative, Judicial and Executive branches all conspire to deprive liberty, have all violated their oaths to defend the Constitution, and the people have failed to change that condition through the political process, then resistance or even revolution are morally justified. But when you say it is ‘moral’ to ‘leave the union’ that does not mean it is as simple as walking away. Think about it. If the government refuses to respect the basic liberties guaranteed by the Constitution, and refuses to abide by the restriction on government action in the Constitution, why would you expect them to simply allow you to walk away. That is why people as diverse as James Madison and Robert E. Lee both said the secession at will in non-sense. It is the state then violating their duty to the constitution. If there are intolerable abuses such as you envision in your reply by an oppressive government, resistance or revolution is the only way out.