To them, they're threatenning the green soldier who is part of the group responsible for destroying armies. I don't know exactly how informed their people are about the forces sent through the portal to attack Earth.
Didn't he call them in from the beginning when he had the "favour to ask"?(Ep05, 15:45) And he balanced politics with his decision. It's not his fault that the princess turned out to be too proud and noob. After the afternoon battle I thought she had a cool head on her shoulders as well at first.The fragile lives that Itami carelessly abandoned as he twiddled his thumbs on whether to send reinforcements from his end of the wall, which includes Rory the imba loli, didn't seem too precious to me.
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edit: rather than portraying something about human life, I think this show about showing off JSDF prowess by matching them with Medieval soldiers.
Americans have their Hollywood movies with a hot intel officer or spy. Japan has anime with elf/loli/mages as eyecandy.