When one is trying to establish what is meant by a term such as 'international security' it is important to consider that such a term will be viewed in numerous ways and to differing levels by many within domestic and national levels and in particular at international or multilateral level. What is important to understand is that by the very flexibility with which 'international security' can be interpreted the methods to which it can be applied can therefore also be wide-ranging and infinitely difficult to define and pigeonhole.
The terms 'international' and 'security' themselves are relatively easy concepts to understand though in terms of international relations the complications that can arise by placing the two terms together is, as the question suggests and as I have implied, one which is riddled with areas of conflict and difficulty. In its simplest of terms 'international security' refers to the secure order in which the nations of the world operate and maintain their multiple relationships with one another, whether that be on an individual stance, in bilateral agreement or within multilateral frameworks and organisations such as the United Nations, Arab League or European Union.
It is by these means that almost all forms of international security are attained and maintained.
In a modern interpretation of international security there are essentially two schools of thought, which dominate global attitudes as to what constitutes international security and that which does not. Although these two schools, liberalism and realism, define the large scope of what we commonly accept as international security studies they in themselves can be broken down into multiple theories within their own understanding of international security, with each section of the main schools presenting differing views of the larger ideologies, e.g. neo-realism, neo-liberalism, radical perspectives etc.
Before attempting to define and understand each...