Often dm's accidently teach players that they don't need to/shouldn't run away. Every time you do the whole "nobody has ever made it out of those caves alive" or a fearful NPC shouts "we need to run away!" and the players push through and succeed anyway you're kinda teaching players that fighting through it is the solution to impossible problems.

If you want players to think running away is an option, you should teach them. Make escaping feel like a win for the players. Consider setting up scenarios where they need to steal something from an ancient dragon's hoard and get out with it alive. Or after the players have gotten whatever they need from a dungeon and they're low heath have a huge strong enemy appear near a passage that the enemy is much too big to fit through.

My group just had to retreat in our last 2E session yesterday. Got ambushed at night, horrible player rolls, plus our GM rolled max damage 90% of the time. After two rounds we had to run and leave the fighter to die to avoid a TPK.

Yeah, it took a while in my group (including me) to actually understand this too. It was like fight until death, but this isn't the best option all the time. Making decisions that values characters own lives brings more depth to the characters.

this is one of the things that makes Lancer so awesome. they have lots of rules for making battles more then a fight to the death including auto retreat conditions for the party

I like the rule in 3rd Age that the party can always and instantly retreat, but the cost is some failure on the track of the campaign. So the bad guys get what they want, a key NPC or item might be lost, etc. Makes it easier to throw challenging encounters at the players without worrying about wiping the party.

I had a great roleplaying moment when I gave the party the chance to surrender when it looked like they might get TPK'd. This is a criminally underrated option that results in some really flavorful gameplay.