This is the 559th most frequent German word.
armee
Army.
Here, 'Armee' is used as a singular noun referring specifically to the military forces of a nation.
Die Armee verteidigt das Land.
The army defends the country.
In this sentence, 'Armee' is again a singular noun, but it refers to the organized group of soldiers under command in a specific historical context.
In der Geschichte wurde die Armee von einem tapferen General geführt.
In the story, the army was led by a brave general.
Here, 'Armeesysteme' is a compound noun where 'Armee' forms the base, referring to the conceptual and organizational varieties of military structures worldwide.