What would happen if a state (RI, purely for the sake of argument) were to secede from the US?

With slightly different geopolitical circumstances I could see Alaska pulling it off, or Hawaii.

In 1865 half of America banded together and seceded as a whole It was http://en.wikipedia.org/wiki/American_Civil_War, but who's counting? I wouldn't worry about nay-sayers that contend Rhode Island, politically cut off from the rest of the U.S., couldn't sustain an independent economy or even feed itself. Rhode Island is bigger than San Marino, Liechtenstein, Andorra and Malta. These countries do fairly well for themselves. Moreover, Rhode Island is not exactly landlocked -- it has a deepwater port, and sits astride the major north-south highway linking Boston and New York City. If were already entering into fictional fantasyland with a secession movement, it's not altogether absurd to think Rhode Island could do just fine all by itself.

The practical answer to your question: The state that claimed it had seceded would face argument from Washington telling it that it hadn't, and telling its government to knock off the bullshit. If negotiations didn't work, the President would order in the Army. Which has happened in the 20th Century, though not directly in response to a claim of secession. http://en.wikipedia.org/wiki/Little_Rock_Nine, a federal court ordered Arkansas to integrate its schools. The Governor refused to do so, and used the National Guard to prevent black students from entering a previously white-only school. President Eisenhower sent in elements of the 101st Airborne. So there's legal precedent for using the Army to enforce federal supremacy over the states. And of course with the Civil War there's also precedent for using the Army to force seceding states to rejoin the Union. So that's what would happen in your hypothetical case.

A state may not secede from the Union. Lincoln had it right: no union of peoples contains the seeds of its own demise in its organizing document. the result would be quick invasion to take over all federal property within the state.

To get around some of the issues during the war, you'd need to pick a state that has lots of in-demand natural resources, ports that could be protected, preferably a border with another country, and people who have a strong sense of statehood. In addition to ian1977's suggestion of Alaska or Hawaii, may I also put forth California and Texas? I think this would be an interesting story! Ok, what if... Democrats take Congress and the Presidency and Texans get mad. Texans secede from the states, and then are forced to make alliances with Mexico (tearing down the fence! speaking Spanish! oh, horrors!) and other oil-rich countries (Venezuela! Bush's friends in the Middle East!) to successfully fight against the States? To me, the interesting part would be the alliances that must be made -- that may not exist now -- and how that would strengthen/weaken/change the citizens of the "new country".

Houstonian: I don't buy it because I don't think the demographics are there in the same way that they were in the 19th century. Just looking at the political spectrum, Texas and California are both politically purple. There currently is no hot-button political deal-breaker that threatens the fundamental economic development of any US geographic region. States are going to become more interdependent and affected by job mobility as time goes on.

Texas is a purple state? Then why do we feel so http://en.wikipedia.org/wiki/Red_states_and_blue_states? Point taken, though, that there is no issue big enough. But, I assumed for the story that the reason for secession is somehow taken care of... maybe an economic issue? Entering speculation just for brainstorming's sake, what if it is Peak Oil time, and Texans have economic reasons for wanting independence? Then, they look back at the oft-quoted Treaty of Annexation and think they have a cause? THEN, let's say, they are forced to make alliances, which may very well result in them changing their minds about many other issues (because they need the alliances)... which could result in The Country of Texas splitting into the five states it was allowed when it re-entered the US? Hmm, sounds like I want to write this book. :)

Houstonian: Texas is a purple state? Then why do we feel so red? Scroll on down to see the HoR and the county-by-county breakdown. The basic problem is that for secession to even be a possibility you need geographic, cultural and economic isolation that just doesn't exist within the 48 contiguous states except perhaps with the Native American Nations who have an extended history of grievances. Now 20 years after Peak Oil time, we might be talking.

Texas is a purple state? Then why do we feel so red? If you look at the cartogram by population in that Wikipedia article's http://en.wikipedia.org/wiki/Red_states_and_blue_states#Purple_States, yes, Texas does look purple. California, however, is pretty blue.