Sub folder to root url rewrting

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Hi,

My Blog is installed in a "blog" subfolder of the root. The root has a htaccess also.

I have been asked to enable an alias / rewrite rule for the domain as follows.

mysite.com/us/blog which points to mysite.com/blog so when the user is perusing the site the us contingent stays in the url even though there is no folder of this name.

The following code in the root htacess works to not break the url
RewriteRule ^us/blog blog/

However when I execute the url in a browser i get the page not found concrete 5 page.

Does anyone know what else I need to do? Is it more rewrites of something to do with the site.config file?