There have been many US-Saudi arms deal in the past, it won't affect the balance in the region.
You guys are seeing this all wrong. It is not a favour the US are doing the Saudis, but the other way around. The Saudis are choking on Oil dollars, they know they have to do the occasional bit to help out the US economy. They are always buying stuff they don't really need to buy some goodwill.
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