How we change what others think, feel, believe and do
Alliances in war
The first task of diplomacy in war is to ensure other countries do not side with the enemy, remaining at least neutral and hopefully sympathetic to your cause.
The next step after making friends is to get them militarily involved. A classic mechanism for this is mutual defense agreements that broadly say 'If you are attacked I will leap to your defense (and vice versa)'. Such agreements can extend across many countries.
Once your allies have joined the fray, the next step is to agree who does what, who commands what and work together as a single coherent unit.
War is an expensive business. Transporting troops and weaponry requires a massive logistics exercise. Every gun fired, every bullet shot has to be paid for and individual missiles can cost millions. Having allies reduces your cost significantly.
War can also be tricky around friendships. Countries who you think are neutral or even friendly towards you may speak out against you. They may even, heaven forbid, help your enemy in any way from intelligence to joining in with troops and weapons. When bystanders are on your side, they at least will provide moral support and perhaps more than that.
Friends in war can and often must be bought and there may be much horse trading with export quotas, technology access and so on. Obligations can ripple down the years and people who
Although alliance agreements are not always honored well, they do lead to a strong defensive position that says to the aggressor 'If you attack me then all of my friends will attack you'.
When joined with allies, political in-fighting can easily muddy the waters as jostling for control leads to arguments about which general is controlling what.
In the 20th century communist-capitalist Cold War, Eastern European Soviet bloc and the opposing Western NATO countries formed alliance groups.
Obligations: British involvement in supporting America in Iraq was probably linked to American support in the Falklands War and even back to the First and Second World Wars.
Leadership struggles: In the Second World War the exiled Charles DeGaulle fought hard to retain control of the Free French whilst American and British commanders decided who would do what.
In negotiations, combine with other parties to provide negotiating strength. Leverage other friendships as well, for example showing how an unsatisfactory conclusion will make many enemies.