This question is stemming from this post in AskReddit:
How accurate is this characterization of Poland in between the World Wars? Were they an expansionist and imperialist nation?
Poland definitely was a nation with an aggressive doctrine in its actions. This was largely due to the actions of Josef Pilsudski, the Marshal and military leader of Poland.
Here is a pure timeline:
1918: With the target of creating a greater Polish superstate, sometimes referred to in historical writing as Intermarium, sometimes Międzymorze (Latin/Polish for "between the seas"), war breaks out between Poland and the Soviet Russian and Soviet Ukrainian Republics (back then, they were legally not yet the USSR), as well as various Ukrainian states, with the Soviets currently fighting the Russian Civil War. The war will see initial Polish success (Pilsudski's Kiev Offensive), then Soviet success, then Polish success again. In 1921, the war ends in a military stalemate, but with Polish territorial gains in the east.
January 1919: After a series of political and diplomatic conflicts along the disputed Cieszyn (Teschen) Silesia area, Czechoslovak units attack Polish lines. As the Polish military was distracted, Czechoslovakia gained the advantage and, eventually, control of the region. Not the "fault" of Poland per se, but still a direct result of their unwillingness to step down in the territorial conflict. To be fair, neither did the Czechoslovaks.
June of 1919: Poland is in diplomatic conflict with the United States and their president, Woodrow Wilson. He basically forces upon them the protection of their national minorities, which make up maybe a third of the country's population. Poland is unhappy, as neighboring Weimar Germany with its Polish minority was not forced to sign such a treaty. As Polish attempts to modify the Versailles Treaty fail, the issue becomes one of national humiliation. Poland would be not really good at protecting the rights of its minority Germans, Ukrainians, Lithuanians and Jews.
17th of July, 1919: The war between Poland and the various Ukrainian states I mentioned ends in Polish victory. (The "Polish-Ukrainian War" of 1918 to 1919)
October 1920: Under the so called Mutiny of Zeligowski, ethnically Polish military units in eastern Lithuania rebel against the Lithuanian government on the direct orders of Polish Marhsal Jozef Pilsudski. They occupy the Vilnius region, making it the disputed Central Lithuanian Republic. This Republic ends up being annexed into Poland after a dubious referendum in January of 1922.
1921: The Peace of Riga ends the Polish-Soviet War and most of the direct and major combat engagements that Poland is involved in.
1926: With a military coup, the fragile Polish democracy comes to an end. The Sejm becomes a mere puppet in Pilsudski's hand, who establishes a not-quite-official military dictatorship with himself running the things behind the scene until his death in 1935. After that, a council around people like Ignacy Mościcki and Colonel Jozef Beck runs the national affairs.
While the dreams of the Intermarium failed, Poland, which had maintained cordial relations with France, still had aspirations to lead a bloc of neutral nations in Eastern Europe, including the Baltic States, Hungary and other countries. These countries would protect one another from the two big players in the region, the Soviets in the east and the (in the eyes of the Polish government) rapidly rebuilding Germans in the west. These dreams were however prevented by the rise of the nazis. Initially friendly relations between Berlin and Warsaw also prevented Poland from joining the Little Entente, which had Czechoslovakia at its member, which had become something of a Polish rival over the mutual border disputes and the somewhat pro-Soviet stance of Prague, while Warsaw viewed Moscow as a strict enemy.
The streak of "Polish expansionism" ends with the occupation by Germans and Soviets in 1939 and the subsequent occupation and establishment of a puppet state by the Soviets after the war.
Czechoslovakia (The Czech Republic), Lithuania and the Soviet Union (today Belarus and Ukraine) all ended up regaining the territory Poland had taken from them in this time frame.
So was Poland militaristic? Yes. Were they aggressive? Yes. Were they imperialistic? Well, it comes down to your personal definition of imperialism. They did not travel around the world and tried to plant flags on African soil, but they certainly had expansionist ambitions, either out of defensive or out of purely offensive and pseudo pan-nationalist reasons. Even Polish minorities, not even majorities, were enough for Poland to consider additions to its potential Greater Poland.
Sources:
Davies, Norman: "White Eagle, Red Star: The Polish-Soviet War, 1919-1920", 2003
Gąsiorowski, Zygmunt: "Polish-Czechoslovak Relations, 1918-1922,", 1956
Stachura, Peter: "Poland, 1918-1945: An Interpretive and Documentary History of the Second Republic", 2004
Timmermann, Heiner: "Das Münchener Abkommen", 2005 (german)