They didn't, as a matter of policy merchant republics didn't tend to fight wars they either couldn't win didn't have to fight. Fighting the horde would be such a war, there are few disputes short of a full-on insult against the Khan that would have brought war to the republics; they were on good terms with the Khans, they sent their tribute and in return they were granted protection and allowed to set up their trading colonies. No need to defend what they gained through prior agreement.