Best cheapest way to increase Defense Index?

Thing is, dont wanna get involved in war levels, so in order to increase in game levels, I need to increase my Defense Index.

I wanna buy something, the cheapest if possible + less soldier/officers needed.

Suggestions¿?
Thanks.

You can choose to move up game levels in a peaceful manner so don't even bother purchasing any weapons. Click on the game level indicator at the top of your screen and then you should see an option to switch to peaceful criteria for game lvls.

Increasing the defensive index is extremely expensive and becomes more so.
the requirements for the defensive index increased and the only way for higher game levels is higher war levels.

The peaceful option is best if you are not really interested in the war game.

Ok thanks.