I've accidentally read "wojna" at first sight, a word I already knew, so I answered 'War is in the city'. This seems kind of unfortunate, as earlier in the lesson, "army" gets translated as "armia", something I'd take to mean (without having done research!) specifically ground forces. Or army in the stricter, military sense that is. Whereas wojsko might also be rendered as armed forces in general?! Certainly not in this context, but in others probably. If this is correct, it might perhaps be added to the tips some day.
In short, not in Polish language – "Armia" works either as a colloquial synonym for military or as a kind of military unit(5th Army = "piąta armia", for example), as immery already pointed.
As for strict military sense, Siły zbrojne Rzeczypospolitej Polskiej(Armed Forces of The Republic of Poland) are divided:
Wojska Lądowe (Land Forces)
Marynarka Wojenna (Navy)
Siły Powietrzne (Air Force)
Wojska Specjalne (Special Forces)
Narodowe Siły Rezerwy (Reserve forces)
Żandarmeria Wojskowa (Military Police)
Dowództwo Garnizonu Warszawa (Warsaw garrison command)
Służba Kontrwywiadu Wojskowego (military counter-intelligence)
Służba Wywiadu Wojskowego (military intelligence)
And Auxiliaries(supply and logistics and so on). So, ground forces in military sense, would be "wojska lądowe" in Polish. ;-)
I'm surprised that nobody has mentioned "The Polish Home Army (Armia Krajowa, AK)". How easily we forget! I believe that there are numerous monuments throughout Poland commemorating the feats of the AK. How on earth can you say that the word "Armia" is not in the Polish language? Haven't you seen those monuments? There is no doubt in my mind that the leader of the Warsaw Uprising, Tadeusz Bór-Komorowski, wouldn't be impressed by Emwue's comments!
I have no idea about English but, accodring to WSJP in Polish
1) duży zespół ludzi wyposażonych w broń, mający zapewnić państwu bezpieczeństwo oraz prowadzić działania wojenne/ big group of people with weapons , with a purpose of providing a country with safety , and conduct military operations
2)pewna liczba dywizji lub korpusów danego rodzaju wojsk pod wspólnym dowództwem /certain amount of divisions or corpses of the given kind of military forces, under common command.
3)wielka liczba ludzi/ great amount of people- colloquially/
1) duży zespół ludzi wyposażonych w broń, mający zapewnić państwu bezpieczeństwo oraz prowadzić działania wojenne = ARMIA
2) określona grupa żołnierzy / certan group of soidiers- colloquially/
3) obowiązkowa dla określonych grup obywateli służba w wojsku - zespole ludzi / obligatory for certain groups of people serve in the army (group of people) colloquially/
As you can see those words are 100% synonymoys in their primary meaning, but the more colloquial meanings are with "wojsko" while more technical meaning is with "armia".
also all things connected to army have adjective - wojskowy