This is a very specific variation on the classic martini, defined simply by the vermouth-to-gin ratio in the glass. 

Essentially, for every 1 part of vermouth the drink calls for 15 parts of gin. For a 100ml / 3.5 US fl oz martini this equates to 1 tsp vermouth topped up with gin or vodka. 

This ratio creates a relatively dry martini. It is not as dry as an extra dry martini or the specific bone dry martini, but it is above average dry, and if a bartender asks you how sweet or dry you like your martinis, this is a very specific, succinct answer. If the bartender does not know this type of martini that is absolutely fine, as it’s a fairly niche request, just remember that it’s a ratio of 15:1.

Origins

So where did the specific name Montgomery Martini come from? 

Field Marshall Bernard Law Montgomery, the 1st Viscount Montgomery of Alamein was a British veteran of both World Wars. He led the Allied forces to victory in numerous fields, notably North Africa where he was known for a key strategy of keeping his own casualties down. 

Strategy

His method was to try and outnumber enemy forces in any engagement with a ratio of 15 soldiers to 1. He would avoid confrontations where this ratio couldn’t be achieved, slipping around the desert and focusing, Sun Tzu style, on opportunistic battles where his target ratio would ensure small, quick wins, which over time added up to strategic victory on the battlefront. 

Subsequently, his highly effective approach has been recognised in martini form, an homage to devastating victories in the form of a potentially devastating drink. 

Given that this is a simple ratio variation of the classic martini, you could garnish it as per usual with either lemon peel or an olive or two. However, to distinguish it from any other martinis you might be pouring, and maintaining a militaristic theme, it might be worth garnishing this drink with a “sword” garnish. Use lemon peel as you would in a classic martini, then shape it into a short but relatively wide rectangle or even oval.

Bend this a thread it through a cocktail stick so that the peel resembles the handle of a sword. The lemon handle should be protruding from the drink for visual effect. 

To be quite frank, however, Montgomery was known to be quite a blunt individual, and presumably accustomed to army life. As such a garnish might not be needed at all. Attack!