I feel for you. I had the same experience. I forgot that battles were restarting and was saving for a Pharma upgrade at the time, so I ended up losing 60k to 80k each in Drugs, Alcohol, and Weapons.

So, should you attack back? Personally, I don't attack others in the game. But, if you do decide to attack, you should only do that if it is worth it and if you have a good chance of winning. That's because if you're only doing to get back at the attacker, you might end up losing more troops in the end. So, as suggested, check out https://simulator.drugwars.io/ and see if your troops will stand against the guy who attacked you. If not, you might have to build your troops first. After that, you can just wait for the person you want to attack to get their resources out.

Should you attack smaller fishies and steal from them? There's nothing wrong with that. That's the goal of Drugwars, after all. They want people to encourage activity, which includes attacking, upgrading, building troops, etc. Otherwise, the game would be pretty boring. But again, if it's not worth it, then why attack? Also, the chances of small fishies having a lot of resources out are small, so you'll be better off spending your time with the big fish.

Since I was attacked and had over 180k of resources (Drugs, Alcohol, and Weapons) stolen, my strategy has been three-fold. First, I upgrade my Storage Silos as much as I can. This way, I can store more resources safely. My goal is to have my resources safe in the 100k level. Next, I build my troops as much as I can. This way, even if people see that I have resources out there, they won't find attacking me a lucrative target. After all, if they have to lose a lot of their troops to get my drugs, they might think twice about it.

And finally, I invest as much as I can on the heist. Heist percentage is pretty high at the moment--as high as that for Drugs Production. So, it is worth it to sacrifice drugs production for the heist, a little bit. Once my safe levels are high enough, I can then save up the resources to upgrade safely. I'll probably be repeating this for a while until something changes in the game again.

So, good luck on your game!