War - International
The laws of the land governs the serious aspects of relationships.
Ultimately all material transactions come under legal jurisdiction, and to various degrees that are dependent upon the particular culture, customs also may come under the jurisdiction of law.
The basic form of the law is different from one culture to another, as is the degree of development of law.
Enforcement of law within a land, nation, or legal area will be carried out in some cultures by the people themselves, or by people specially assigned the duty, or in more developed societies by 'police'. The more complex and developed the society, the more developed its laws and legal system, and so the agency and agents who apply or enforce the law will also commensurately more developed. When Judge Robert Peel founded the police force in Britain (the 'Peelers') we saw the first police force in the world charged with and specialising in the task of enforcing the law.
In the United Kingdom the police were set up on a regional basis and areas corresponding to counties and cities established.
Law against Law
Separate nations through different histories, cultures, and traditions arrive at different sets of principles and laws.
Whenever two countries or nations have disagreements and come into conflict with one another, immediately a conflict between the laws of one will come into conflict with anther.