So in my world a portal suddenly opened on a field. The portal leads to a planet full of monsters. Why would the people in my world attack and defeat the evil planet. What resources would there have to be for there to be strong motivation. This is in a semi-modern world. For example 1800s or 1700s. If it matters the portal was opened by evil magicians.
The same old reasons.
Take their loot and bring it home. Put it up in your city to impress the folks. Or melt it down and make coins out of it, with your face on them. Maybe a monster on the flip side so people can chuckle because they got their loot taken.
Take their land and grow stuff on it. Sell that stuff, or eat it. Or sell it to hungry monsters who used to get their food there until you showed up. Use their payments for reason #1.
Take the monstery people and make them work for you. They can build a triumphal arch in your city depicting you showing up and taking their stuff. Once that is done you can have them do other jobs you need doing.
Convert their monstery people to your religion. This is compatible with points 1 thru 3.
Kill their monstery people because they came thru the portal to your world with #1,2,3 or 4 in mind.
If the monsters happen to resemble abominations from any of the major religious texts, then the portal might be mistaken for a gateway to the darker half of the afterlife. With such a mistaken conviction, the living heirs of less than saintly loved-ones might invade hell in hopes of liberating their recently damned deceased. Monsters who happen to die during that liberation would just be collateral damage.