The tribes of Reuven and Gad said: "We will build pens for our livestock here [east of the Jordan] and cities for our children" (Bamidbar 32:16). Moshe responded: "Build cities for your children and pens for your livestock, and that which comes out of your mouth you shall do" (ibid. 24).
The whole story of the give-and-take between Moshe and the tribes is a wonder. As soon as they came with the request that they receive the land east of the Jordan, Moshe jumped on them as if they had refused to fight on behalf of their brothers, when in fact they immediately explained that they were prepared to come and fight. Why had Moshe not considered that possibility?
Apparently, until explained otherwise, it was clear to Moshe that the tribes of Reuven and Gad would not be willing to leave their many children behind. After all, they had the mitzva of educating their children to tend to. Thus, if the families were staying behind, certainly those of army age were going to stay behind as well.
The good news in their clarification was also the very bad news. They were planning on leaving their children to go fight, and they even mentioned their interest in protecting their significant herds before mentioning their concern for the protection of their children, putting the less important first. On one hand, Moshe could not object on a formal level on behalf of Bnei Yisrael, since they were keeping their part of the deal, but Moshe could not refrain from putting them in their place, correcting the order and putting the children before the cattle.
One can still ask: what difference does it make what order the tribes mentioned the building projects. The important thing is what they would do, and who knows if they still would not build the pens first? Yet Moshe stressed the words "and that which comes out of your mouth you shall do." In other words, specifically that which one says does impact. It is hard to imagine how big of an impact a statement, even a slip of the tongue, can have.
"They built Nevo and Ba’al Meon, Musabot Shem … and they called with names the names of the cities that they built" (ibid. 38). Rashi explains the words Musabot Shem – they changed the names of the places that referred to idolatry to new names. When they came to Eretz Yisrael, the nation knew that they were not continuing the legacy of the nations which preceded them. They knew that they were going to have to create a life appropriate of the Israelite nation. They knew that Hashem’s Name was called upon them, and so they changed the names of the places around them. However, that was not enough, as they had to live in a manner that the name proves to be appropriate.
In these days as well, we have changed names and created names. Villages and towns that were once occupied by other nations are now full of life from immigrants from all corners of the world. We must remember what it says in the beginning of the parasha: "… he shall not profane his word" (ibid. 30:3). The daily life must prove the names appropriate.