Frightening information about the history of the Pledge of Allegiance is at (with shocking historical photographs).
For fascinating information about symbolism see 
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The Nazi salute came from the USA

Another source of the "Roman salute" myth is the book "The March of Fascism" by Giuseppe A. Borgese (1937).  The book's makes the claim but with a complete lack of evidence.

Here are the two paragraphs from the book from which the "Roman Salute" myth grew (and note that there is no reference to any actual ancient source, only general assertions without support): "The gesture of the raised right arm, which was to be sooner or later the Roman and, unbelievable but true, the German salute, had been picked at random from classical museums, from gestures of Græco-Roman orators and rulers, and perhaps also from the medieval romance of Carducci. In antiquity it had been occasionally an attitude of oratorical vehemence, or of command, or even of pardon. It may also have been seen, occasionally, as a salute from the distance, which happens nowadays as it always has, whenever people at the railway station or on the pier bid farewell to departing friends. It never had been the ordinary salute in the streets of Greece and Rome, where the free citizens shook hands or affectionately clasped each other's wrist, while no doubt the slave, meeting his master, saluted with the raised right arm, almost to show that his hand was disarmed and his obedience defenceless. A salute of slaves; such indeed was the gesture of Fiume to become, sooner or later, in Italy and Germany. D'Annunzio and the Fiumani liked it because....." (You'll have to buy the book to read the rest).

The lack of reference to the U.S. pledge of allegiance and flag salute suggests that Borgese was unaware that the National Socialist Francis Bellamy caused the straight-arm salute to be used extensively in the U.S. from 1892, and the salute was still being used in the U.S. when Borgese wrote his book in 1937.