Based on some preliminary thoughts; Sinsic government at the time would've definitely sided with the Vaguzian government, which was considered more 'legitimate' in their Eyes, though would've otherwise kept itself directly outside the war. Sinsu could've joined the fighting if the communists targeted the embassy and killed everyone there who were stuck and didn't manage to evacuate in time, including the Ambassador, and effectively declared war on the Republic? One could then develop that further and include a hostage situation + negotation where the Communist rebels demanded that some of their jailed comrades in Krimeon would be set free with a plane and cash, to which the Sinsic would've refused etc.
If this somehow wouldn't work, Sinsu as mentioned would've sided with the Vaguzian government in any case, providing monetary support and allowing volunteers to fight in the war, for example. I'm open to any suggestions as well.