1st off, my salute to you for serving in the US military to help Afghanistan, regardless if you will be able to read this post here or not, but can be an example for naive others whom are bullied by so called 'gangsters'.
Next, you did the right thing instead with your communication to them that at the least convinced them to back off. Spore is a Rule of Law nation, & everyone, including foreigners are protected by the law. Just ask them which secret society they belong in, then tell them you too, as guest of Singapore, belong to a society known as 999 - Singapore's Police dept. There is no need for violence, and in such events should violence be initiated by others, civil lawsuits will come into play, with evidences from cameras.
It is not for you alone, but every guest & Singaporean citizen as well. Our Founding fathers & forefathers had done well to institute the Rule of Law in Spore, for decades, & had so called triad leaders rotting in prisons even now. They know what is in score for them, & thus they backed off.
In our hard won multi-racial country, the golden rule RULES - Hurt/Harm none & not be hurt/harmed by anyone else, or else unnecessary repercussions will only ensue.