Defence against whom ?
The UK and US are engaged in defending their countries in Afghanistan, Iraq and elsewhere. In spite of the US having arguably the world's most effective and certainly the world's most expensive military, there was a traumatic attack on home soil against which all their might was useless in that not until months afterwards the reaction all came in flailing around: Now travellers worldwide are put through inconvenient and expensive screening in reaction to something that was purely aimed at the US.
If Scotland becomes independent then they are unlikely to want to emulate the UK's worldwide ambitions, after all independence will finally consign history of the Empire to the past. Granted, much of the No support is based on sentimentality regarding the past (note, it is otherwise not much use for the future) but it is mostly down to fear of change - the Nos may win, but the Yes also may win. As by most measures of social performance the UK is among Europeans rather low on the lists (infant mortality, life expectancy, education, health care efficiency and efficacy, social equality, women's participation in management and politics, peacefulness, happiness, etc., etc.), by breaking away Scotland would have a chance to outperform the UK rather than be locked/trapped within it (but celebrating that the UK was once a great power - which can still be celebrated even if it is divided). It will however take considerable effort for Scotland to reach the top of the lists from such a low start, although of course it would be possible. In this regard the UK's current task may appear nigh impossible - a smaller country/unit has a better chance, both Scotland and the remaining UK. That is the way I see it: Both resulting parts stand to gain, and I am convinced they would be on very good terms.