> If you want to argue that crashing into the atmosphere would be an inevitable result of the trip

That's not what I'm arguing FFS, I'm saying that'd be the result of going with the fastest possible trip, which would still leave the target significant time to set up countermeasures.

Here's the rest of the paragraph of my original answer since you apparently stopped at the first period:

> If you want to actually deliver your payload in a useful manner you'll need twice that at the very least. 2+ months is a long lead time, and the engine you'll need will make the craft impossible to miss.

The point is that no matter how you slice it, "the difference between building on Mars and building it in orbit" is the target gets a warning several months in advance in the former case, a few minutes or seconds in the latter. One gives ample opportunity to set up and deploy countermeasures, the other not so much.

So why not build the countermeasure before you build it in Earth orbit? what's the difference then.

[deleted some pointless back and forth]

Because there's no way you'll be able to detect the launch and deploy your countermeasures, even assuming said countermeasures actually work which is unlikely: it's doubtful we can counter ballistic missiles as-is, a payload starting from orbit can launch much closer to the target (rather than half a world away) and is not fighting gravity for half the trip.