The first manifestation of the inflexibility of the conquered, a manifestation of the invincible belief in final victory, was bringing into being in capitulating Warsaw an underground combat organization, which was given a name the Service for the Victory of Poland. Its first commander, for a while though, had become General Michał Tokarzewski-Karaszewicz. In December it transformed into the Association of Armed Struggle (Związek Walki Zbrojnej - ZWZ), controlled from Paris by Gen. Kazimierz Sosnkowski, appointed by Gen. Władysław Sikorski, then Supreme Commander. After the fall of France the command of ZWZ was held by Gen. Sosnkowski from London, until his political breakthrough with the Supreme Commander and the Polish prime-minister; Gen. Sosnkowski had to resign because he opposed the Soviet-Polish alliance, concluded on 30 July 1941 by Gen. Sikorski and the Soviet ambassador Ivan Maisky. In February 1942 ZWZ was transformed into the Home Army (Armia Krajowa - AK), the first commander of which became Gen. Stefan Rowecki (Grot), one of outstanding Polish junior commanders; he belonged to those officers of the pre-war Polish army, who from the beginning were impressed by issues of armoured units. When Rowecki was arrested by Gestapo, what happened in summer 1943 due to a betrayal, the commander of the Home Army became Tadeusz Komorowski (Bór).

Simultaneously with the Home Army were arising in whole Poland many resistance groups, which were created by various political groups, or just by more venturesome people. The merging of the large number of organizations, usually acting in a complete isolation, cost a lot of work and lives. Many commanders, despite of the cause, refused to subordinate to a central headquarter. They attempted, surely noble but nażvely, to fight and to win with their own, mostly very tiny, forces.

Basically the Home Army was disposed towards the general uprising only during the final stage of the war, when an occupation regime would collapse; it had to intercept the country's territory and to hand it over into hands of the government coming from exile. Besides the Home Army was carrying out intelligence; among others it signalled German preparations to the war against the Soviet Union and had gained and handed over to the British plans of German missile weapon V-2, which was designated to overpower Great Britain towards the close of the war. A strong detachment under the cryptonym Wachlarz (Fan), brought into being to support a future uprising by destruction of enemy's communications out of Poland, since April till December 1942 was demolishing railways, transports and storages in eastern territories, then its soldiers reinforced other specialized units of AK. There were also committed many outrages on senior German commanders and officials, among others the chief of the police and SS in Warsaw, Kutschera, was shot. About 1600 railway transports were destroyed and about 2000 Gestapo agents were liquidated. 

The partisan warfare as the main form of the fight was adopted from the beginning by the People's Guard (Gwardia Ludowa - GL), created in January 1942 as a combat organization of the Polish Workers' Party. The first partisan detachment of GL, led by Franciszek Zubrzycki (Mały Franek), went from Warsaw to the region of Piotrkow Trybunalski on 15 May 1942. The detachment was lost in battle but by December 1942 there were 29 fighting GL's groups and detachments, and by autumn 1943 - as many as 60. Then started the formation of battalions. Later the People's Guard became the core of the People's Army created by the decree of just being created the National Council of Homeland. To it's commander was appointed General Michał Żymierski (Rola), a veteran of the First World War and a graduate of the French École Supérieure de Guerre. Within the year 1944 the People's Army formed 10 partisan brigades and about 20 smaller units, which were disorganizing German communication routes between the Reich and eastern front as well as struggling with German occupation system. In April 1944 in the Soviet Union was created the Polish Partisan Headquarter, in van of which was Gen. Aleksander Zawadzki. It resided in Shpanov near Rovno and organized the training and transfer to Poland of sabotage and diversion groups.

Since 1943 the Home Army as well formed partisan detachments too. Their cadres were constituted mostly by the officers sent from Great Britain where they got a versatile training in diversion and then they were dropped with parachutes to Poland. That is why they were called cichociemni (silent and dark) - because they usually landed in Poland by silent and dark nights. Partisan detachments of the Home Army played a serious role in many fights, carried out either by own forces or with the detachments of the People's Guard and the Peasant Battalions, a combat organization of the Peasant Party, among others in Vilnius, province of Zamosc, on Tanew river and in Sola Forest. In the summer 1944 the command of AK undertook an operation Burza (Tempest), the goal of which was to destroy the German defence in face of the Soviet army's offensive and to intercept the power in Poland in name of the government in exile residing in London and led, after the death of Gen. Sikorski, by Stanisław Mikołajczyk. The culminating and the most tragic episode in the history of the Home Army was the Warsaw Uprising. It was the greatest and the longest uprising operation of a kind in the field of human conflicts. However ill-prepared, launched prematurely and without a co-operation with regular allied forces it finished with the most horrible defeat in Poland's history.

A particular tragedy of the war had become the fate of the beforehand sentenced by Adolf Hitler to extermination Jewish population of occupied countries. The biggest part of it, about 3,500,000, lived in Poland. As early as in autumn 1939, promptly after the Polish campaign, hitlerite administration began to drive the Jews away to isolated urban districts and provincial centres. Similar ghettoes were subsequently created in other conquered countries. The biggest however were those in Warsaw and Lodz, of about 500,000 and 300,000 people respectively. With time invaders began to transport to Poland the Jews from other European countries. They were in for death in one of the extermination camps, mainly in gas chambers of a large, called later a "death factory", death camp near Oswiecim (Auschwitz). An inborn to contemporary Jewish communities passivity and subjection to state authorities were maintained by the Germans with the assistance of traitors and ambiguous promises to spare their life in exchange for a hard work for the Reich and release the remnants of the possessions. In an inexpressible crowd of ghettoes and poverty of the most inmates thousands were dying of starvation.

The whole Europe was tormented by the barbaric occupation; every day of the long war cost the nations of Europe twenty thousand killed or murdered. Everywhere the help to the Jews was imperilled by punishments, even up to a death penalty. Nevertheless the help to the Jews had grown to a wide scale. In Poland there was brought into being the Council for Aid to the Jews (under the cryptonym Żegota); in exile the governments of the occupied countries appealed to the powers to undertake warning and repressive measures, which could stop the process of annihilation of the Jewish nation. Unfortunately, in vain - simply nobody believed the reports from occupied countries; they were deemed exaggerated, and the most reluctant were the influential Jewish circles in the United States.

But even in passive crowds harassed in ghettoes had to and did arise various forms of resistance. In Warsaw ghetto in October 1942 was created the Jewish Combat Organization (Żydowska Organizacja Bojowa - ŻOB), which with an aid of AK and GL started a military training of volunteers, gathering the weapons and persecution of traitors. When a next stage of deportations to extermination camps began and when the news came about just resolved, complete liquidation of the rest of the Jews, on 10 April 1943 the 600-men strong ŻOB struck against hitlerite troops marching in ghetto. The uprising of course had no chance to win, but it proved to the Germans that even the most hunted down will not submit to hitlerite verdicts. Against the insurgents was used the artillery, demolishing house after house, and eventually the poison gas, when they fiercely fought in a system of underground bunkers. The uprising extinct on 8 May, when its commander Mordechaj Anielewicz died. Polish resistance saved some survivors, others were put to death at the camp of Treblinka. On 16 May, in the report to his superiors, SS-Brigadeführer Jürgen Stroop (after the war sentenced to death for war crimes) reported that the Warsaw ghetto is no more. The military onrushes though to a less scale took place in other Polish towns - Czestochowa, Bedzin and Hrubieszow as well as in the USSR in Vilnius, Belostok, Kremenets, Kletsk and Glubokoye.

Large proportions assumed the resistance movement on German-occupied Soviet territories. By the end of the year 1941 about 3000 partisan groups and detachments were already acting there. The Official History of the Great Patriotic War of the Soviet Union 1941-1945 says that

To a wide scale extended activities of underground forces in Yugoslavia, where after all the ground conditions were favourable to such a form of fight. Already on 26 June 1941, several weeks after the defeat of the royal army and shortly after the German attack on the USSR had been created the General Staff of the National Liberation Partisan Detachments of Yugoslavia, led by Josip Broz (Tito), a general secretary of the Yugoslav communist party. Within July a partisan warfare enveloped all the regions of the country but Macedonia, where it flared up in October. Then were liberated western Serbia, substantial part of Croatia and some regions of Bosnia and Montenegro. On 19 September invaders struck back aiming at the main partisan grouping, about 15,000-men strong and located between Uzice and Valjevo south to Belgrade. In action against partisans took part over 100,000 men, among others the troops of a puppet Croatian petty state. But they failed to destroy the partisan grouping, which managed to plough through the encirclement ring and retreated to Bosnia.

In 1942 had been carried out a reorganization of Yugoslav partisan forces. The large units, called proletarian brigades, were created. In November formally came into being the National Liberation Army of Yugoslavia. Partisan detachments were transformed into regular military units with their own headquarters, logistics, supply services and intelligence. The actions of those units were also developing to an extend of regular operations. Invaders had carried out several large anti-partisan offensives and tens of smaller counter-attacks, but none of them secured a goal.

In May 1944 the Yugoslav national liberation army numbered over 300,000 men and controlled over a half of the country. In the summer the Germans, with a help of collaborationist detachments, undertook yet another serious effort and even managed to seize a Bosnian town Drvar, where quartered Tito, his staff and allied military missions. But the headquarter evacuated to Vis island, whereas the seizure of an empty town was a Germans' success as dubious as short-lived. Soon they had to flee from there since the Soviet armies approached in September the Yugoslav frontiers, and the whole satellite system in Balkans was on the decline towards a ruin.

Such were in short the resistance activities in three countries, the most actively engaged in the struggle. But an underground movement developed also in Czechoslovakia, Scandinavia, Low Countries, France, Balkans and Italy.

The fighters of the Norwegian resistance movement managed to destroy a German-built works producing so-called "heavy water", necessary to set nuclear reactors in motion.

About 70,000 combatants filled by the beginning of the year 1944 the ranks of the Greek national liberation army, which controlled about two thirds of the territory of the country.

In the end of August 1944 brought up the Slovak National Uprising; albeit two months after insurgents had to abandon their centre in Banska Bystrica and to retreat to the mountains, they inflicted Germans the losses as big as 40,000 men. 

Throughout Europe were carried out the outrages on German governors like Reinhard Heydrich in Prague, Wilhelm Kube in Minsk or Franz Kutschera in Warsaw.

With aid to allied forces were hastening partisan brigades in Italy and France. In France so-called maquis, namely partisans, had delivered men for regular troops formed in already liberated country.

Overcoming incredible difficulties to the struggle against hitlerism were raising the Germans themselves. Many conspirators came from the German Communist Party, crushed by Hitler during his seizure of power. In summer 1943 in Krasnogorsk near Moscow came into being the National Committee Free Germany, created by German immigrants in the USSR as well as some imprisoned officers and soldiers of the German army.