One of the few benefits to lockdown is that I have never had so much time to read, and got through so many books. Being enthusiastic about modern history, I feel like I have reached some conclusions on foreign policy and defence that are worth recording.
Depending on your country's capability to adopt the philosophy, the best approach seems to be that of former US President, Theodore Roosevelt: "Speak softly and carry a big stick", i.e. focus on diplomacy and encouraging good relations, but have the potential to use force if needed, which helps you to be taken seriously.
More obviously, I believe in putting great effort into maintaining close ties with your allies, and doing what you can to support them, economically and militarily, for mutual benefit and aim to uphold an environment around the world that is favourable to your country and its outlook and values. What has been shown over the centuries is that it's highly unlikely that all countries can adopt identical outlooks and values. As much as capitalism and democracy are most favourable from the standpoint of Britain and other likeminded countries, it seems unrealistic to expect all countries to share equal enthusiasm for these concepts. Significant tolerance is therefore important to ensure widespread good relations. Whilst the way that other governments operate, in terms of their political systems and treatment of certain rights, may run counter to our outlook and be distasteful, significant (though not unlimited) tolerance is valuable and practical. For example, there can be great debate over getting the correct balance between freedom and stability. Having spoken to people from Vietnam and Singapore, they have said that certain freedoms their countries may lack (relatively) perhaps allows greater benefits in terms of the stability of their countries, which I feel should be respected, even if these countries may prioritise some values differently to the way myself and many other British would.
Note that I said that tolerance of difference systems should be significant but not unlimited: two principles I like to think my country, and other likeminded countries, tend to support in the modern era are that the mass killing of a country's people by their own government, and any invasion or attempted conquest of another country against the population's will, are both intolerable. These actions should encourage condemnation and consideration of military involvement, ideally on a collective basis with other countries. One would hope that the United Nations would back this stance, adding legitimacy.
Although, as mentioned earlier, the emphasis should be on diplomacy and not military aggression, the latter can be justified in the two above international scenarios, as well as in defence of allies and, of course, self-defence.
