which tactic is correct?
defend then conquer or conquer then defend

Depends on the map, amount of players, current income, income of other players, and 100 more conditions...

try and do both, maybe.

What x said.

There's a lot more to tactics than just attack and defend tough.
Choose your picks wisely, anticipate where you can expect your enemies, pay attention to the order of your attacks and transfers, use cards when necessary etcetera.

It's generally a good idea to keep expanding even when you've encountered an opponent.
But make sure to spend enough armies to defend your bonuses and break his if you can.

If you want to know more, you can ask your opponents for advice after the match, or read some articles on the wiki, there are a few good articles there.
Hope that helps.

thx