Which country has smallest border?

nice question

tho the answer may depend on what you mean by country

as well as what you mean by smallest

& also what you mean by border

believe it or not

but monaco at about 4400 meters has the smallest border of any country in the united nations

tho the vatican city

which is also often considered a country

has an even smaller one at about 3200 meters

& the best answer is probably to just give you your choice of these 2

but there are smaller borders

which however are shared by countries that have other borders too

& thus cant be the one country nor provide the one border you appear to be seeking

still

it may be fitting to mention that the shortest known land frontier of any kind is shared by morocco & Spain at a place on the coast of North Africa called penon de velez de la gomera

& it is so small that it may practically disappear at high tide in a storm tho it is normally about 100 meters long

&

a fixed boundary segment between Bangladesh & India with an unvarying length of about 30 meters surrounds the smallest enclave in the world

which is named upan chowki bhaini

&

a very possibly shorter frontier may also exist in Africa at the confluence of the zambezi & chobe rivers where 4 countries converge so closely together that it is not even possible to say with certainty which 2 of them share the resulting miniborder

most people consider that it must belong to Botswana & Zambia because these 2 countries are joined by a ferry & observe normal border formalities at the ferry terminals

yet the best available maps indicate that the 2 countries which really adjoin in this location are not botswana & Zambia at all but rather namibia & Zimbabwe

but in either case this frontier is quite possibly the smallest border in the world also