[sliken]

Assuming A) that FTL is impossible and B) the galaxy isn't a mono culture. The established players will likely have an mutually assured destruction agreement not to fight among themselves. Defending against near light speed attacks is basically impossible... at least for planets.

Civilization inhabiting whatever their favorite star systems in the galaxy would have to deal with a few problems. One is that the size of the galaxy (100k light years) galaxy wide quite difficult. Another problem is that civilizations can go from pre-radio (before they are easy to detect) to mass energy conversion (allowing a decent fraction of light speed) within a few 100 years. One likely strategy would be to place sensors across the galaxy that would detect problematic aggressive/warlike civilizations and eliminate them.

So civilizations that are peaceful, grow slowly, don't ruin their planet, and don't set off nukes in their atmosphere might get a pass. Maybe even an invitation into the galaxy wide community once they figure out anti-matter/mass conversion.

However the ones launching nukes might just get eliminated before they become hard to contain. If the typical window from pre-ratio to a growing thread takes a minumum of 200 years they might well spread a sensors to be within 100 light years of any planet. That would need something like 250k to cover the milky way. That way the problem is eliminated within 200 years and before they can spread far enough to be hard to contain.