Inlanders vs. Outlanders

The Inlander and Outlander conflict exists as a result of climate changes pushing people to move into more distinctive locations where their colonies are less impacted by weather.
For example: hollowed-out mountain, underwater, underground. Essentially, using the Earth's terrain (or can also be natural sites that offer a lot of protection, such as possibly behind a waterfall) as protection for various environmental disasters, a group of people have decided to move "into" the Earth, thus deemed Inlanders.
Outlanders are just as the title suggests, are people are decided to remain living on the Earth's surface.
A distinct feature that separates the two groups are:
Inlanders are more technologically advanced, because a) the majority of people who moved inland are scientists, and value science and technological advancement as a dogma of their existence b) it requires a certain level of technology to maintain the Inlander nations (i.e. for not limited to air filter system/HEPA and HVAC, maintaining sunlight-resembling day-and-night clock system, inland greenhouses, etc.). Thus, each Inlander nation likely contain their own self-sustaining hydroelectric plants and agriculture and such.
Note: a main trade and export to Outlander colonies are power and hydro.
Outlanders are more devoted in religion, they believe that through their faith in their local deities, they will be guided to live safely on the surface.
Between the nations and colonies, there are however borderlines drawn, and this serves as a first point of conflict as Inlanders have been known to invade Outlander territories to expand their nations (this may be because Inlanders have limited space for their nations due to the nature of the cities). Another point of conflict is Outlanders view Inlanders are people who do not respect the Earth and mother nature, thus leading to dismal conditions of the Earth as it is (as traditionally, technological advancements has also lead to various environment disasters), despite the Inlanders' efforts to reverse previous generations' faults and damages to the Earth. Lastly, Inlanders resent Outlanders, because Inlanders also has a focus of advancing/propagating the human race (a decline in viable birthrates due to various ecological disasters) whereas Outlanders does not share the same perspective (they believe more that when the times is right, viable birth rates will recover, as the Earth recovers). Thus, Inlanders view Outlanders are lazy and holding back population growth (more so because of the declining birthrates and aging older population, they more people to replenish the workforce).
In summary, the Inlanders and Outlanders with their respective beliefs struggle to co-exist peacefully.

Belligerents

Inlanders

Outlanders

Strength

More technological advancements

Religious beliefs has allowed them to navigate the Earth's surface more easily

Casualties

Objectives

To advance forward and bring up viable birthrates. To maintain a steady population with enough living space for all

To win back the Earth (either through converting Inlander people to their beliefs or more extremely, eradicate opposing Inlanders)