Lithuania was a major frontline through most major European wars: Medieval crusades, Napoleonic wars, World War 1/2, Cold War.
If you are interested in modern (19th-21st century) warfare, these are the top 10 locations you should check:
1.Look down into an authentic nuclear missile shaft at the Plokštinė nuclear missile base, now doubling as a Cold War museum.
2.Spend entire day exploring the 19th-century Fortress of Kaunas that once subsumed the entire city with the aim to defend Russian Empire from invasions from the West. Many of its massive forts are now abandoned, although two converted into museums. There are also entire districts of barracks (some abandoned, some repurposed) in Šančiai and Freda, both districts also having military cemeteries (Šančiai one still in use).
3.See the massive private collection of Atomic bunker museum (Soviet districts of Kaunas) consisting of intelligence (KGB), military communications and civil defense devices. Everything is housed inside an authentic Cold War-era nuclear shelter. The museum helps to understand the atmosphere of fear that existed in Soviet-occupied Lithuania. The regime feared World War 3, while many people feared the regime itself.
4.Check the War museum (Kaunas New Town) is both as a repository of weapons, history of Lithuanian armed forces and a patriotic shrine, with a war memorial in its basement and sculptures of national revival heroes in the surrounding square.
5.Try the hands-on approach at Museum of Military Vehicles in Vilnius (Antakalnis), a repository of military vehicles (especially Soviet).
6.Search the forests for reconstructed makeshift Lithuanian partisan bunkers to feel the hopelessness of standing against a massive Soviet army in what was the modern Europe's longest guerrilla war (1940s-1950s). One such bunker is available south of Panevėžys, near Pašiliai bison reserve.
7.Check the Memel-Nord battery on the northern beach of Klaipėda. Built as a Nazi German WW2 military installation it now serves as a small bowdlerized private museum about the lives of German soldiers on the Eastern Front (open in the Sundays).
8.Use the Klaipėda ferry to visit 19th century German Sea Fortress that guarded the entrance into the city (currently serving as the Museum of the Seas). Military ships may often be seen on the other side of the Klaipėda channel (bring binoculars).
9.Go to the Aviation museum in Kaunas (Aleksotas) that has many images of interwar Lithuania's massive air force, which owned 117 aircraft (at the time Lithuania was also a producer of fighter planes). It also has a few more modern aircraft parked on the outside.
10.Seek the now abandoned early 20th-century military bunkers which were built on Lithuanian countryside by foreign regimes that dominated it after new mobile warfare rendered the fortresses obsolete. Polish installations still stand in Vilnius suburbs (from the Polish rule of Vilnius era, 1920-1939) while entire Lithuania is spanned by DOTs - defensive bunkers that Soviets constructed in Soviet-occupied Lithuania in 1940-1941 to defend their "new western border". The line of defense was not complete before Nazi Germany attacked. One easily accessible DOT stands in northern Palanga.