Learn German: Wir kämpfen gegen die Trockenheit dadurch, dass wir das Feld oft bewässern. - We fight against the drought by irrigating the field often.

It translates to "by [doing something]". We use it to express the means or method used to achieve a goal. The word dadurch sits in the first clause pointing forward, while the dass clause explains the actual method (irrigating the field). Because it is a dass clause, it forces the conjugated verb bewässern to the very end of the sentence.

Could I use indem instead of dadurch, dass?

Yes, absolutely! indem wir das Feld oft bewässern means the exact same thing and also sends the verb to the end. The two structures are mostly interchangeable, though dadurch, dass puts a slightly stronger emphasis on the method.

Why do we use gegen die after kämpfen?

The verb kämpfen (to fight) is commonly paired with the preposition gegen to mean "to fight against". The preposition gegen always forces the following noun into the accusative case. Since Trockenheit is a feminine noun, we use the accusative feminine article die. If you wanted to say "fight for", you would use kämpfen für, which also takes the accusative case.

Are the words Trockenheit and bewässern related to other common German words?