Who said anything about immigration?

Let's start with abandoning science funding, abandoning investments in and tax credits for renewable energy sources that are the future. Then there's applying political pressure to academic institutions to drive even more researchers away, abandoning the civil service and science/reason-based governance. Move over to the medical sector and put a dangerous anti-science nut in charge, kill off funding and research.

You pull out of international organizations, trade deals, and treaties. You throw temper tantrums and tariffs around, flip flopping day to day and making it impossible to predict the costs of doing business. You antagonize the rest of the world and give them a constant stream of reasons to stop doing business with you, leaving you isolated, weaker, and poorer.

Then, sure, let's go briefly to immigration. America has been great because it has been where all the smartest people want to be. Our political and academic environment caused the smartest people around the world to want to be here, and the US benefited massively from their contributions and inventions.

So you build a culture demonizing anyone not a specific shade of white. You destroy visa programs. You send thugs to universities to harass people and make them unwelcome. You tell students and researchers who went home to see family that they can't come back to the US. Then you send thugs into cities to terrorize and murder people. So you give all those brilliant scientists even more reason to leave the US, take their contributions with them, and never return.

That's how you kill America.