What is justifiable about russia’s position/entrance into the war?

Keep in mind the following. Regarding the Zimmermann Telegram, Mexico completely ignored the note and didn't take it seriously. Given that, was a declaration of war by the US upon Germany justified? Regarding the Lusitania, it was tragic, but it also cannot be denied that Germany had declared a general war zone and the use of unrestricted submarine warfare surrounding the British Isles, and US ships willfully ignored that warning.

Even if the entry of WW1 was unjustified, did it still have economic or political benefits? Keep in mind, the US had already established itself as a world power in the Spanish-American War, the sun was about to set on the British Empire regardless of the US' entry into WW1, and the prosperity of the 1920s in the US may have no link to the outcome of the War.

So, what do you think? Was the death of over 100,000 Americans worth it? 

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The correct answer is b