Game Advice

I'm playing South Canada in a game (532,839 if it helps) on day 53 (as of this post).

I allied with Southern USA early in the game (day 16).  Together we control the Americas and Greenland.

I have also gone on to conquer Iceland, Scandanavia and will have North Russia under my control shortly.

On day 30 Southern USA declared war on West Algeria.  At that point in the game USA was 4th overall and Algeria 5th.  Algeria was 5th in army size at 7% and USA 7th at 4%.  I was 2nd in army size, also at 7% and 3rd place overall.

Southern USA went to war of his own accord and didn't consult with me, his ally, which is fine as I figure any ally may chose his own wars without have to check with one another.

I now have a problem and it is this...

Southern USA is losing this war BADLY.  As of this post Southern USA is now 6th overall, West Alegeria in 4th.  Army size of US is 7th at 3%, Alegeria army size is 4th at 15%.  I am 3rd overall and 2nd in army size at 23%.

I have stepped in once to help repel a West Alegeria invasion of Southern USA.  Once repeled I was back at peace with Algeria.  FYI, we are only at peace, nothing more.

Southern US keeps sending troops to attack and keeps losing.  The only reason Algeria has not attacked back is due to the alliance between myself and Southern US.

Algeria has asked if I will continue to defend Southern US and/or if I would ally with him.  I take alliances very seriously and don't want to betray Southern US yet I feel I will be forced to defend the Americas on my own due to USA's actions.  Alegeria won't hold back forever.

I know I could just join the fight against Algeria but I'm not super keen on doing that.  As an ally should I continue to help defend Southern USA's borders from invasion due to his poor playing?  I don't think it would be wise to allow West Alegeria to have a foothold in the new world.  Any input and advice on what to do would be appreciated, I'm sure it's an easy decision on the outside looking in.

P.S. Sorry for the long winded post.

Well, if anyone is interested I decided to stick by my ally.  After typing the above I realize that was probably the thing to do.