Would Arizona harass US citizens?
The US government is claiming Arizona's tough new law to tackle illegal immigration would cause the harassment of American citizens, ignore humanitarian concerns and interfere with foreign policy.
So it is taking the state of Arizona to court to try to stop it ever happening.
Critics of the Arizona law, which is due to come into force at the end of this month, claim it is racist. The government's complaint doesn't go that far but it does say the state is overstepping its authority and suggests it is blundering into a delicate area.
The Arizona law would mean that police officers would be able to arrest people on "probable cause" of a public offence "that makes that person removable from the United States".
The US government's official complaint says "it will cause the detention and harassment of authorized visitors, immigrants, and citizens who do not have or carry identification documents specified by the statute" arguing this would "result in countless inspections and detentions of individuals who are lawfully present in the United States".
The government's wider argument is that immigration policy is the rightful preserve of the federal government, which has to balance many competing interests. It says it can't be done at the level of a state. It says the Arizona law is only interested in reducing the problem of illegal immigration ("attrition"), and, it says:
"It will altogether ignore humanitarian concerns, such as the protections available under federal law for an alien who has a well-founded fear of persecution or who has been the victim of a natural disaster. And it will interfere with vital foreign policy and national security interests by disrupting the United States' relationship with Mexico and other countries."
No doubt there will be an agonised debate about states' rights and some will be keen to portray this as intrusion by central government. But there is a much more basic problem. The United States, nation of immigrants, has long see-sawed between welcoming new blood and pulling up the drawbridge.
President Barack Obama suggests much is being done to keep illegal immigrants out, insisting last week:
"Today, we have more boots on the ground near the south-west border than at any time in our history. Let me repeat that: We have more boots on the ground on the south-west border than at any time in our history. We doubled the personnel assigned to Border Enforcement Security Task Forces. We tripled the number of intelligence analysts along the border... The southern border is more secure today than at any time in the past 20 years."
But there's a perception that whilst this may be the federal government's job, it is not doing it. While Mr Obama has signalled it is a priority to introduce new rules about immigration, nothing has happened. His strong speech has not been matched by any move in Congress. Anyhow, if there was a new bill the president has made it clear it would centre on turning illegal immigrants into citizens, rather than finding new ways of making their life more difficult and throwing them out.
It is the apparent failure of the federal government, seemingly frozen by the difficulty of the problem, that has goaded Arizona into action.
I’m Mark Mardell, the ³ÉÈË¿ìÊÖ's North America editor. These are my reflections on American politics, some thoughts on being a Brit living in the USA, and who knows what else? My 
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