Hi, I'm also a Polish scout troop leader in the US, which hufiec are you from? We also happen to be doing the theme of WW1 this year. Anyways one of my zastępowi found this post when planning his zbiórka and asked if it was me so I decided to reply.
For the causes, I am doing less games, but I am tieing it more into the entirety of the year.
For militarism, I am giving them little plastic toy soldiers that represent their armies, they get these in relation to how many people from their zastęp are at zbiórka, winning different games between zastepy...etc. These toy soldiers represent their "armies." Later in the year, I plan to introduce a "war system" for the zastępy to compete using these soldiers. There will be a map of Europe and every week they will be able to lottery for a certain city on the map. Each zastep can decide how many soldiers they want to put into the lottery and the zastep whose soldier is picked, gains that city (more soldiers you put in, the higher chance of winning.) This is a fun little detail added into the zbiorki, but can add a lot to the overall theme.
For nationalism, I made each zastęp one country. I gave each zastęp the flag of their country and told them that they will receive punktacja for adding the culture of that country into their zastęp. I have found this works better than just games because they will get really creative with it. So far one of my zastępy has learned the whole Russian anthem, another has come dressed up in some cultural garments during one zbiórka...etc.
For imperialism, we are doing a canoe biwak on the Wisconsin River in two weeks. Every so often, I plan to have the harcerze dock on islands and race to put the flag of their zastęp up (thus symbolizing imperialism/the scramble for Africa...etc.) I am also involving small games based off different concepts, for example the Boxer Rebellion in China...etc. If you would like to see the whole program, just message me and I can send it to you.
Regarding alliances, I am still trying to come up with a fun way to incorporate it, but I hope this helps you out!
For those from BSA/ Non-Polish Scouting reading this:
hufiec = group of troops centralized around a certain region, for example there is one on the East Coast, one in Detroit, one in Chicago, one on the West Coast.
zastępowy = leader of the "zastęp"
zastęp = group of 4-6 scouts that work together as a team for the whole year (also known as a patrol)
zbiórka = meeting of scouts where we do all of these activities, for example we have ours on Saturdays
punktacja = points
biwak = short camping trip
harcerze = scouts