This song plays as the legend of Fire Maidens in the story was told. It has 2 version for the countered factions, which are complete opposite in context.

Wehrmacht military style / US army that speaks German / Japanese school in a Spanish town somewhere in France /
Speaking Japanese, using Yen as currency yet can't read Kanji / miko-nun which serve under Shinto-Christian religion ...... etc.

Yeah, this show's a mess indeed.

Either you find it with the given hints in plain-text (K-ON! reference, the big fella known as the 6-legged spider tank "Takemikazuchi", a relic from the past)

or the hidden links such as "trumpet", the color theme of Wehrmacht uniform, anti-war sign, Amazing Grace.

The end of Part 3
Here you go