In May 2015, the US Supreme Court asked the federal government for advice. Nebraska and Oklahoma were upset about neighboring Colorado's 2012 decision to legalize marijuana, saying that they had a hard time tracking bootleggers bringing the drug back into their own states. The justices wanted to know: Should they hear the case?

After all, under federal law growing, selling, or using marijuana is still illegal.

On Wednesday, Solicitor General Donald B. Verrilli filed his brief on the increasingly tricky issue of marijuana legalization, technically outlawed as a Schedule I substance under the federal Controlled Substance Act. Yet 23 states have approved it for medical treatment, and another four, plus the District of Columbia, have legalized its recreational use as well.

No, thank you, said Mr. Verrilli, the federal government doesn't want the Supreme Court to look at this issue.

Hearing the case, "essentially that one state's laws make it more likely that third parties will violate federal and state law in another state – would represent a substantial and unwarranted expansion of this Court's" jurisdiction, Verrilli told the High Court. According to the brief, the Court should entangle itself in inter-state disputes if the issue "would amount to casus belli [cause of war] if the states were fully sovereign."

It certainly doesn't look like Nebraska is ready to declare war on Colorado over marijuana bootleggers.

The Supreme Court may or may not heed the Solicitor's advice. Either way, this case is unlikely to stem the legalization movement, which promises to divide states and continue to create interstate dilemmas until a decision is made at the federal level. For many observers, it's not a question of when, but how.

The state marijuana quandary has existed since at least 2009, when the Department of Justice said that prosecuting drug use legal under state laws would not be a priority. In 2013, then-Attorney General Eric Holder said the federal government would not challenge states who legalized the drug if they could regulate it satisfactorily.

Oklahoma and Nebraska say such an already-tricky frame — giving nodding approval to activities that are illegal at the federal level — isn't working for them, even if it works for Colorado. They say marijuana crimes are up, and blame it on pot coming from across the state border. Nine former Drug Enforcement Administrators, who filed a brief with the Court, agree.

"The state of Colorado has created a dangerous gap in the federal drug control system enacted by the Unites States Congress," Nebraska Assistant Attorney General Ryan Post told the Court.

"This kind of not-in-my-backyard legal argument, if successful, could be deployed against any other states seeking to follow Colorado's example — and could therefore impose a massive obstacle to legalization at anything short of the federal level," CNN analyst Prof. Stephen I. Vladeck told the cable TV network when the Court reached out for a recommendation last spring.

Nationwide, momentum seems to be growing for legalized pot use, especially as states consider potential revenue they could collect on legal, taxable cannabis. The "War on Drugs" has done little to stop marijuana use, and many figure they might as well take financial advantage of what some say is a relatively low-risk drug.

Recreational use is now legal in Washington, Oregon, Alaska, Colorado, and in the District of Columbia, and likely to come up in 2016 votes in a handful of other states, as well, such as California and Massachusetts.

Opinion polls indicate that resistance to legalizing the drug is fading, especially among younger Americans. Some 58 percent of Americans say it should be legalized, including over 70 percent of young adults.

As long as legalization is handled on a state-by-state basis, however, difficulties like Oklahoma and Nebraska's are likely to continue to pop up. But some people say that treating individual states' marijuana laws as experiments – from which the rest of the nation can watch and learn – is a wise move for now, especially as lessons are learned about its health effects.

Legalizing pot is "not an end-of-days disaster," the Brooking Institution's John Hudak told The Christian Science Monitor in February, but not much else is certain.

“Right now, this is an exercise for the marijuana industry to make money,” said Kevin Sabet, cofounder of the San Diego-based Smart Approaches to Marijuana (SAM), which opposes legalization. “My biggest concern is that we’re creating Big Tobacco 2.0, and we’re allowing them to market a drug that has a big risk for the developing brain to young people.”

But others think that play-it-by-ear approach is unsustainable, as more and more states declare cannabis legal: "Something’s got to give, and it’s going to be federal prohibition," the Drug Policy Alliance's Stephen Gutwillig predicted one year after Colorado gave pot users the go-ahead.

Ultimately, the gradual state-based approach might lead to a Supreme Court decision, as it did with gay marriage: same-sex couples legally married in one state wanted their unions recognized in all of them, somewhat similar to marijuana users' wish, but pot use is less compelling as a "fundamental right."

Others think marijuana will follow in alcohol's footsteps: in 1933, after nearly 14 years as a "dry nation" in name, if not deed, Congress voted to repeal the 18th Amendment.

In 2014, an editorial series from The New York Times called on the government to "Repeal Prohibition, Again.":

The social costs of the marijuana laws are vast. There were 658,000 arrests for marijuana possession in 2012, according to F.B.I. figures, compared with 256,000 for cocaine, heroin and their derivatives. Even worse, the result is racist, falling disproportionately on young black men, ruining their lives and creating new generations of career criminals.

According to the American Civil Liberties Union, marijuana use is roughly equal between white and black Americans, but black users are 3.73 times more likely to be arrested for possession.

"Policy diffusion is often associated with an S-shaped curve," Prof. Andrew Karch of the University of Minnesota told Pacific Standard when its writers were attempting to predict the day marijuana might become legal nationwide. "The initial process of adoption is slow and steady, then the pace picks up, and finally it levels off."

The writers concluded that roughly 30 states would have to legalize marijuana before federal law reaches a tipping point. Their best guess of when that might happen: 2021.