It's all "States rights trump Federal rights" when it suits them.
Similarly, it's all "Federal rights trump State rights" when it suits them, too.
The reality is they're not arguing in good faith. Period.
I'd love to understand it, would someone please explain this to me? I'm not from or living in the US :(
The civil war in the US was about slavery. A lot of people (mostly conservatives) argue that it wasn't actually about slavery, but instead a vague notion towards the relative autonomy between states and the federal government, also called "states rights". The idea behind this meme is pressuring a straw man conservative into answering "what state rights" with the implied answer being "to allow the ownership of humans".
Well it sure isn't for the state right to do their own substance control (e.g. letting Coloradans smoke pot and take shrooms when they want.)
Nor was it about the state right to marry gay couples (e.g. Hawaii and Michigan).
And as soon as they can, they're looking to ban abortion Federally, so it's not about the state right to regulate family planning and reproductive health services.
During the slavery / abolition crisis, it was admitted by quite a number of pro-slavery statesmen (including my own ancestor, Jefferson Davis) that the war was entirely about the necessity of institution pf slavery to sustain the economy of the states below the Mason-Dixon line.
In the US, the federal government's powers have historically been very limited, with many of the day-to-day laws that affect the average citizen's life being decided at the state level instead. This has led to conservatives pushing their unpopular policies at the state level whenever they can't succeed at the federal level.
In the early 1800s, this included whether slavery should be legal. Slave states insisted it was their right to keep slaves and the institution of slavery while also passing a federal law called the Fugitive Slave Act which would compel the governments of free states to capture and return people who escaped slavery back to the slave states.
Events like the hero John Brown's raid on Harper's Ferry with the intention to start a slave revolt and his subsequent execution and martyrdom and Lincoln's election to the Presidency as the leader of a (at the time) very antislavery Republican party scared the slave states into seceding and starting the Civil War to preserve the institution of slavery.
After the war's conclusion, slavery was restricted to prisoners only, which is the primary reason why the US has such a high incarceration rate, and why that rate is disproportionately high among black people specifically. The military and government officers who had led and fought for the pro-slavery secession were pardoned and allowed to return to public life.
A few years later, the Compromise of 1877 withdrew federal troops from the slave states which allowed the white institutions of those states, often led by those same pardoned slaver officers, to use murder and terrorism to roll back the political gains black people made in the years immediately after the war and also to begin a campaign of historical revisionism claiming that the cause of the Civil War was federal imposition on the slave states' rights rather than the institution of slavery, despite each slave state's declaration of secession plainly stating at the time of their secession that the cause was to protect that institution, and despite the fact of the greatest federal imposition on states' rights leading up to the civil war, the Fugitive Slave Act, being in their favor.
Ever since, whenever conservative controlled states, which are predominantly the same states which seceded for the cause of slavery, are prevented from doing some horrible thing which violates their peoples' rights like racial segregation or denying gay people equal treatment under the law, they go back to crying about states' rights.
