Why did Romans believe it was acceptable to conquer other countries?

Answer 1

The Romans wanted to conquer so many lands because the other lands had things that they wanted The Romans wanted to conquer so many lands because the other lands had things that they wanted

Answer 2

Firstly in antiquity there were not countries in the modern sense. There were kingdoms, territories inhabited by ethnic groups and named after these groups (they are often called tribes, but they were often ethnic-wide collections of tribal groups) and city- states.

Secondly conquering other peoples or countries was acceptable until the early twenty century. Ancient, medieval and early modern history is littered with battles between powerful states fighting each other and trying to conquer each other's territories and with instances of empire building. It happened all over the world. The Europeans colonial powers conquered most of the world between the 16th and the 19th century. In the 1880s it was considered perfectly normal for the European powers to agree to carve up Africa and rush to conquer as much land as they could.

Thirdly, imperial expansion during the Roman Republic was not due to a desire for or a grand design of conquest. Apart from Julius Caesar's deliberate conquest of Gaul, expansions were the result of Rome getting drawn into wars. She won and this resulted in annexations.