Given that space isn't owned by any one country, and Starlink is a US company, whose services are used by the US government, there's a very good chance there would be military retaliation.

Look up Kessler syndrome for an answer as to what may happen if starlink satellites were attacked, although I think I recall people more knowledgeable than me stating this orbit is low enough where it may not be a permanent issue. Nations only control their airspace up to the Karman line which is 100km up. Starlink satellites are well beyond that.

It would be hard to do? How much would that affect the general use of starlink for users on other parts of the world?

Only two countries have demonstrated air launched rockets that can destroy satellites on orbit, the USA and Russia. There is good speculation that China has built anti-satellites satellites, but no one is aware of any actual proven test.

Here's the USA's anti-satellite rocket being launched on its one and only test:

Now, lets assume that all 3 countries decide they want to attack Starlink satellites at once with all their weapons. Perhaps they destroy 30 satellites in total. As of November 2025 the Starlink network surpassed 10,000 satellites in orbit. As for replacing the lost satellites, a single launch places 25 to 28 satellites in orbit at a time. Within the next 24 hours 25 more Starlink satellites will be launched:

In 4 days, another launch is occurring that will place 24 more Starlink satellites in orbit.

source

So destroying a few dozen Starlink satellites might cause a slight blip in coverage for maybe a few minutes tops in specific narrow geographic locations, but only for a little while until replacements move to positions.

Very hard to do. You need to be able to launch to the same orbits as the satellites and Starlink uses thousands of satellites, almost all of which will at some point fly over those countries. So they'd need to shoot them all down.