Nationalism is not patriotism. Whereas patriotism is the love of one's country, nationalism is the divinization of one's country, to worship one's nation as a god. During World War One, the various European nations were transmogrified into the divine instruments of God and His wrath on earth, with catastrophic consequences. The Liberal Protestant clergy were one of the chief culprits, and here is one such blasphemous prayer from a German pastor:
Our Father, from the height of heaven,
Make haste to succor Thy German people.
Help us in the holy war,
Let your name, like a star, guide us:
Lead Thy German Reich to glorious victories.
Who will stand before the conquerors?
Who will go into the dark sword-grave?
Lord, Thy will be done!
Although war’s bread be scanty,
Smite the foe each day
With death and tenfold woes.
In thy merciful patience, forgive
Each bullet and each blow
That misses its mark
Lead us not into the temptation
Of letting our wrath be too gentle
In carrying out Thy divine judgment.
Deliver us and our pledged ally [Austria-Hungary]
From the Evil One and his servants on earth.
Thine is the kingdom
The German land.
May we, through Thy mailed [sic?] hand
Come to power and glory
[As cited in Philip Jenkins, The Great and Holy War: How World War I Became a Religious Crusade (New York, NY: Harper One, 2014), 13]
Loving one's country is good, but never, ever treat any country or government as divine or semi-divine, as having absolute authority over the souls and consciences of men. Such is idolatry, and idolatry can have real-life consequences, as the First World War and the Second World War have shown us.