You start with your official nation's default land. to expand you must either Declare war and expand or colonize empty territory, how? well you have 2 colonist slots to use, sending a colonist to the land you want to annex takes one turn and expansion takes 5 turns. also you can't colonize outside the map.
There are multiple ways to start a war:
1: Use a Causes belli to declare war, these are: crossed territory illegally, Diplomatic insult, Has claims on certain land, Economic embargo, or failed armed insurgency.
2: invade for no reason.
3: issue an armed insurgency, doing this is will cause patriots to begin a revolution against a neighbor, it's strength will depend on your influence on said neighbor, along with that there is a 50% chance of said neighbor not discovering it was you.
So how are battles decided? well each unit is divided by points, whichever army has the most points wins the battle. points: Army: Light infantry: 1, Heavy infantry: 2, Light tank: 2, Medium tank: 3, Heavy tank: 4. Navy: D class: 2, C class: 3, B class: 4, A class: 5. Air: Light bomber: 2, Light fighter: 1, Heavy bomber: 3, Heavy fighter: 2.
Building units and handling maintenance requires money of course so Economy would defiantly be included. every tile you have gives you 2 gold while unit maintenance is always 1. you can issue and Embargo on a nation which has a 50% chance of Causing all tiles to only give 1 gold for 5 turns. if you fall in debt your Units lose 1 point each and there is a 50% chance of a tile being absorbed to a neighbor/become unclaimed.
I've mentioned most of them so here is the rest:
Ground units move 1 tile per turn, ships travel across the sea after 2 turns and yes Ground units can travel by sea the same way along with air units, and air units travel 2 tiles.
Light infantry: 3 gold (3 turns)
Heavy infantry: 4 gold (5 turns)
Light tanks: 5 gold (5 turns)
Medium tanks: 6 gold (7 turns)
Heavy tanks: 7 gold (10 turns)
D class: 4 gold (4 turns)
C class: 5 gold (6 turns)
B class: 6 gold (8 turns)
A class: 7 gold (10 turns)
Light bombers: 3 gold (3 turns)
Light fighters: 3 gold (3 turns)
Heavy bombers: 5 gold (5 turns)
Heavy fighters: 5 gold (5 turns)
Ground units can only participate in Ground battles of course with Naval forces fighting in the sea and on the coast with Air units being able to fight anywhere.
Influence can help you become friends with someone or take them over, it's simple, being in the same ethnic groups, or alliance give you 5 and 1 influence respectively, you can also increase influence by using diplomacy which no matter what besides war increases it by 1. Use influence points to start a revolt or give your ally free units to help them. Although it only works on certain spaces (say you are Germanic and a Romance country takes a Germanic space, you get influence On that space.) a unit cost the same amount of influence as it costs with money but doesn't require waiting However they lose one point (unless they already only have one) In battle.
Ethnic group are mostly just there for influence but can actually be more useful then that, on any turn you can activate Ethnic offensive mode for 5 turns which causes land gains to take 3 turns instead of the usual 5 you also get Ethnic militants on all of your spaces in an ethnic region you have land in and use them as temporary soldiers with 3 battle points each and ethnic cleansing in a space takes only 5 turns instead of 10. It of course ends after 5 turns of use and takes 30 turns to use again, all ethnic militants would disappear and land gains would go back to normal. You can also try to destroy an ethnic group which replaces it with your main ethnic group but causes high chances of revolts And takes 10 turns.