Although Poland and Polish-Lithuanian Commonwealth (I'll be using the first term to denote both entities, as well as post-1918 Poland) was witness to many military campaigns, vast number of these were localised and usually comprised of border conflicts or aimed at capture bigger or lesser part of the territory of the neighbouring lands, as during the invasion of Lower Silesia by the Bohemian prince Bretislav in 1038 or attack on the same area by German king Henry V in 1109. The same can be said of the wars with Teutonic Order in 14th and 15th century that were not invasions but rather power struggles and territorial conflicts. Since the mid-15th century southeastern Poland becomes a common target of Tartar incursions but these were also localised and rarely extended beyond Ruthenia and Podolia. Contrary to what I found to be quite commonly believed, Germany, Holy Roman Empire or any German principality for that matter was a rare invader, especially after fragmentarisation of the Holy Roman Empire while the majority of hostile invasions were carried by Crimean Khanate (or Tartars in general) and Russia (Duchy of Muscovy and Russian Empire).

The three main reasons of Poland being the arena of multiple wars and invasions, namely its location, size and politics. It is quite obvious that the country lies in the center of Europe (a geographical centre of Europe was until 1772 within territory of Poland) and for almost its entire history it bordered with several large neighbours who were sometimes resorted to rather aggressive politics. It is worth remembering that besides constant presence of Germany (and Holy Roman Empire earlier) to the west and Russia (earlier Kievan Rus) to the east, Poland also bordered with Sweden to the north (across the Baltic Sea and in Courland) and both Crimean Khanate and Ottoman Turkey to the southeast. Strained and often hostile relations with kingdom of Bohemia resulted in numerous conflicts along southern borders that cesaed in late 14th century, although this was moderately off-set by rather amiable relations with Hungary and the natural obstacle that was the western part of the Outer Carpatian Mountains.

On the other hand, good relations with Hungary sometimes put Poland at odds with kingdom of Bohemia, Hungary's rival in the area. Also Moldavia, located nearby, was a political target of Poland, Hungary, Russia, Crimean Khanate and Ottoman Empire what often led to open conflicts in 15th and 16th century. Likewise, the coalition with Lithuania that will later culminated in the formation of the Polish-Lithuanian Commonwealth put Poland at odds with Russia, greatly interested in expansion to the east and conquest of other Ruthenian lands, largely controlled by Lithuania. The Turkic and Mongolian people inhabiting from Crimean steppes were more or less a constant threat to the southeastern Poland since the first incursion of Cumans in 1092 until the end of 17th century (Crimean Tartars were common allies of Ottoman Empire, although occasionally they were also allying with other parties, including Poland).

In the first century after the admission of Poland to Christendom and its recognition as the sovereign state, its first rulers tried to firmly establish their sovereignty and expand the country, what led to numerous conflicts with Germany, Kievan Rus and neigbouring land of Obotrites and Veletes. Internal tensions in 1030s resulted in large-scale uprising against new government that was used by Czech duke Bretislav I and subsequent retaliation of Polish king Boleslav II in 1060 who also fought Pomeranians, Ruthenians, Hungarians and Bohemians in numerous conflicts that later resulted in retaliatory strikes, most notably by Pomeranians, Czechs and Moravians in early 12th century, during the reign of Boleslav III. The fact that king's brother vied for Polish crown and allied with foreign powers only exacerbated existing conflicts. When Boleslav III died in 1138, he divided the territory of Poland between his sons, starting the tumultuous period of provincial divisions, significantly weakening the state that was officially united again only in 1320 with the coronation of Vladislav I.

This central location played an important role in some conflicts, with the earliest being the Mongol conquest of Europe that stopped near the Polish-German border in 1241, 1259 and 1287. This was repeated again during Napoleonic Wars, as Polish territories (then freshly adjoined to Prussia and Russia) were located obviously in the way of Grand Army invading Russian Empire and in 1939 when Germany expanded East executing its Lebensraum policy. The fact that borders between Germany, Austria and Russia lied in the formerly Polish territory also meant that a substantial part of WW1 conflicts were also fought there, but these cannot be technically construed as 'invasion of Poland'.

Political problems became only exacerbated in late 16th century, when Poland official adopted elective monarchy, where any member of the royal families, domestic or foreign could become a candidate for the crown. This, as one could imagine, resulted in elections of foreign monarchs that sometimes pulled Poland into power struggles between members of the foreign royal families and into additional open conflicts, such as Swedish-Polish wars in early 17th century during the reign of Sigismund III of Vasa dynasty who tried to introduce Polish ideas of religious freedom to Sweden and also create personal union between Poland and Sweden what was met with stark opposition of Charles IX. Such tensions culminated in the War of Polish Succession in the years 1733-1735 where several European powers were struggling to put their candidate on Polish throne. Substantial number of potential and actual conflict sources meant that Poland was unable to successfully defend on all fronts, especially given that its imposing area was not matched with country's population (especially because southeastern areas were populated rather sparsely) and whenever Poland was engaged in a conflict with one or two of its neighbours, others were often trying to exploit the opportunity by attacking on another front, like e.g. Swedes did in 1655, after Poland was weakened by large-scale Cossack uprising and was still engaged in conflict with Russia or Ottomans who attacked in 1672 after Poland emerged victorious yet devastated from the war with Sweden and got involved in another conflict with Cossacks aided by Crimean Tartars.

Poland officially ceased to exist in 1795, its whole territory divided by Prussia, Russia and Austria, being partially resurrected in the form of Duchy of Warsaw (French puppet state) and Kingdom of Poland (de facto protectorate of Russian Empire). This situation added another form of conflict, namely insurrections aimed at achieving partial or even total independence. The first one, led by Tadeusz Kosciuszko even before the last partition and subsequent November Uprising in 1830 and January Uprising in 1863, fought primarily with Russia ended in utter defeat however. I'm mentioning them as they were important military events that would add to the impression that Poland was more often than not involved in struggle with foreign powers.