If you doubt Britain needs a written constitution, listen to the strangely unbalanced discussion broadcast by the BBC last Friday.  

The Today programme asked Lord Guthrie, formerly chief of the defence staff, and Sir Kevin Tebbit, until recently the senior civil servant at the Ministry of Defence, if parliament should decide whether or not the country goes to war.  

The discussion was a terrifying exposure of the privileges of unaccountable power. It explained as well as anything I have heard how Britain became party to a crime that may have killed a million people. 

Guthrie argued that parliamentary approval would mean intelligence had to be shared with MPs; that the other side could not be taken by surprise ("do you want to warn the enemy you are going to do it?"), and that commanders should have "a choice about when to attack and when not to attack".  

Tebbit maintained that "no prime minister would be able to deploy forces without being able to command a parliamentary majority. In that sense, the executive is already accountable to parliament". Once the prime minister has his majority, in other words, MPs become redundant. 

Let me dwell for a moment on what Guthrie said, for he appears to advocate that we retain the right to commit war crimes. States in dispute with each other, the UN charter says, must first seek to solve their differences by "peaceful means" (article 33). If these fail, they should refer the matter to the security council (article 37), which decides what measures should be taken (article 39).  

Taking the enemy by surprise is a useful tactic in battle, and encounters can be won only if commanders are able to make decisions quickly.  But either Guthrie does not understand the difference between a battle and a war - which is unlikely in view of his 44 years of service - or he does not understand the most basic point in international law.

Launching a surprise war is forbidden by the charter. 

How Britain Became Party To A Crime That May Have Killed A Million People