Montenegrin issues

The Austro-Hungarian army attacked Montenegro on 6 January 1916 and in less than three weeks the country capitulated; King Nikola I went into exile. The occupied land was run by a military general government, and from 1 March 1917 two field-post stamps – 10 and 15 heller – overprinted 'K.U.K. MILIT. VERWALTUNG MONTENEGRO' were sold there. Holding the mountainous country took over 40,000 troops, twice as many as in Serbia.

Sources: Wikipedia: Austro-Hungarian occupation of Montenegro, Wikipedia (de): Postgeschichte und Briefmarken von Österreich, Stamp Encyclopedia: Montenegro – back of the book issues

A further, much larger set of field-post stamps overprinted 'Montenegro' was prepared for the country. The end of the war overtook it, and it never went on sale.

Sources: Stamp Encyclopedia: Montenegro – back of the book issues