[img]http://i94.photobucket.com/albums/l100/Halcon_24/SU-25%20de%20la%20FAP/A-37ySU-25CeasrAugusto.jpg[/img]

[hider=A 37 is not really common anymore]A-4 skyhawks (Brazil [2], Argentina [22]) and F-5 (Brazil [43], Mexico [9]. Chile [9, replaced with 39 F-16s] Honduras [6]) would pose a significantly bigger threat.

Venezuela also operate a hefty number of Su-30MKIs.

Odd-note:

Peru opted to put A-37s [10] and Su-25s [18] in the same airbase (replacing 32 Su-22 Fitters).

Current A-37s in S.America (flyable):
13 - Columbia
4 - Ecuador
9 - El Salvador
10 - Honduras
10 - Peru
3 - Uruguay
Total: 49[/hider]

If we knock Argentina, Brazil, Chile, and Venezuela off the list of potential enemies (as they don't employ the A-37 at all), then the pool favors A-37s and Su-25s.
-In a way, the fact they are flying such common planes (and more than 3) actually narrows-down who is financing their war.

/over-thought

Although this does mean that [url=http://en.wikipedia.org/wiki/IAI_Kfir]a surprise[/url] could be in order.