Re: Islamically can you join a national army...
Not sure why you did not choose 4:102-104 since those would have strengthened your case.
Several verses in the Quran speak of God's creation doing Iqaamat-is Salaah...
أَلَمْ تَرَى أَنَّ اللَّهَ يُسَبِّحُ لَهُ مَنْ فِي السَّمَوَتِ وَالْأَرْضِ وَالطَّيْرُ صَفَّتٍ كُلٌّ قَدْ عَلِمَ صَلَاتَهُ وَتَسْبِيحَهُ وَاللَّهُ عَلِيمٌ بِمَا يَفْعَلُونَ
(as per Yousuf Ali) Seest thou not that it is God Whose praises all beings in the heavens and on earth do celebrate, and the birds (of the air) with wings outspread? Each one knows its own (mode of) SALAAT and TASBEEH. And God knows well all that they do.
I untranslated the two words for a reason. Granted it is their own mode, but is it in any way unnatural or ritualistic (i.e. I am asking you how are they "performing" their SALAAT and doing TASBEEH)?