Just to say that Finland joined Germany in attacking the USSR after the USSR attacked Finland in June 1941. Moscow had a choice, Helsinki did not.
The process by which Finland ended up in the Continuation War took 15 months and began immediately after the Winter War. Stalin enforced a peace that was somewhat harsher than the already harsh terms the Finns had agreed to in Moscow on 12 March 1940 (the war ended the next day), demanding
de facto war reparations and moving the border somewhat further west than what had been indicated on the map of the eastern frontier accompanying the peace treaty. In the spring of 1940 Finland attempted to secure an alliance with Sweden and Norway, but this would not do for Moscow (neither for Berlin for that matter, for different reasons): Molotov claimed it would be a revanchist war alliance for the purpose of attacking the USSR, and would result in the annulment of the peace treaty and thus the continuation of the hostilities that had ended on 13 March 1940. Soon Norway fell but the Finns made another attempt at allying the Swedes in the autumn of the same year. Certain circles even proposed different variations of political union between the two countries. Pressured by Moscow and Berlin, the Swedes again said no.
While this was going on Finnish communist agitators, now free from prison thanks to the peace treaty, and funded and directed from Moscow, organised various protests and acts of sabotage, while shouting declarations that soon the Finnish government will be history and Soviet rule will be in its place, and that they are backed by the USSR. In August 1940 Göring persuaded Hitler to send an arms merchant to Finland to secretly bolster Finnish military capabilities against the prospect of a renewed Soviet attack. As a part of this deal was also a transit treaty in the same vein Berlin had signed with Stockholm that same summer. The Finns accepted, and in effect, to quote the history books, Hitler had extended the German eagle's wings to protect Finland against the Russians. I've subtitled a related dramatisation from an excellent historical series called
Sodan ja rauhan miehet [Men of War and Peace] from 1978 here, which may be of interest:
View: https://vimeo.com/870938484Persuaded by Göring, who had pro-Finnish leanings and connections through his brother-in-law, the Swedish Count Eric von Rosen, Hitler has sent Lt. Col. Joseph Veltjens to Finland to offer the selling of arms to the Finns and the signing of a transit treaty allowing German troops going on leave to Germany and back to their stations in northern Norway through Finnish territory. In effect this will not only bolster Finland's ability to defend herself from a renewed Russian invasion, but also effectively extends Germany's protection to Finland by virtue of German soldiers travelling through the country, even if the amount of Germans is infinitesimal. Veltjens arrived in Finland on 17 August 1940 and met Mannerheim the next day as well as the day after that. It is possible that he also met Ryti, though my personal understanding with the information I have is that unlike in this documentary drama, in reality Mannerheim made the call to accept the transit treaty without consulting Ryti.
In November 1940 when Molotov met with Hitler in Berlin, his no. 1 agenda on a list of demands to Germany was in effect to resolve the Finnish question as per the secret protocol of the Molotov-Ribbentrop Pact from 23 August 1939. In effect this meant asking for free hands to liquidate Finland, a task Stalin had been forced to put on hold in March 1940, likely due to the international situation involving Franco-British plans against the USSR under the guise of aiding the Finns. Hitler rebuffed Molotov, stating Germany could not tolerate another war in the Baltic Sea area. In the coming months a number of Finnish-German meetings of a military nature took place and in May 1941 the Finns were informed of the plan to invade the USSR. However the Germans presented the matter in such a way all the way until 22 June 1941, that they've sent Stalin an ultimatum demanding the restoration of Finnish and Romanian territory, the independence of the Baltic States and the Ukraine, etc. In reality no ultimatum was ever sent, and the story about it was a ruse to cover up the fact that the Germans were
certainly going to attack the USSR. The Finns, and to my understanding the Romanians, Hungarians etc. as well, were told that the attack will commence if Stalin
rejects the ultimatum. In other words they weren't told that the Germans will certainly attack the USSR, only that it is a likely possibility and that their allies (both
de jure and
de facto) should prepare for it in conjunction with the Germans, which they did. The Finns enacted a general mobilisation on 18 June 1941 and the field army was in position in a defensive posture near the border about a week later. The Russians began hostilities against the Finns at 6:05 am on 22 June 1941 by shelling and bombing, with the Finns returning fire at certain locations from 8:15 am onwards. The Finnish government also issued a note to the Soviet embassy in Helsinki inquiring the reason for these hostilities, which went unanswered. On 25 June the VVS began a bombing campaign, which on the first day involved some 540 Soviet aircraft, about half of which were fighters, against population centres across Southern Finland. The Finnish Air Force shot down some 27 VVS aircraft in Finnish airspace without loss. The same day PM Rangell issued a government bulletin to Parliament which gathered to discuss it at 7 pm:
The air raids against our country, the bombing of open cities, the killing of civilians, have proven more clearly than any diplomatic estimates, the Soviet Union's stance on Finland. It is war. The USSR has renewed its attack with which it tried to extinguish the Finnish nation's ability to resist in our Winter War of 1939-1940. Like then, we now too rise to defend our country [...] This beginning of hostilities by the USSR against our country forces us to defend ourselves with all available means. Today our new defensive struggle has begun.
The next day President Ryti confirmed that a state of war with the USSR has begun.
View: https://youtu.be/tltS1vH26vs?t=947Timestamped, see 15:50-17:05.
Now all that being said, make no mistake, the Finns were committed to Barbarossa. By 25 June they'd already mined the coasts of Estonia and inserted deep-reconnaissance men into East Karelia in preparation for the push there. Finnish air units had been ordered to stand by for the possibility of both defensive and offensive operations. Two German formations, IIRC a division and a brigade, were marching or about to march through Finnish Lapland and two German mountaineer divisions had crossed over from Norway to Petsamo on 22 June. German bombers flying from airbases in East Prussia were flying through Finnish airspace to mine the waters near Kronstadt and stopping over in Finland on the return trip to refuel. German forces were however forbidden from carrying out strikes
from Finnish soil into the USSR until Finland was at war, or otherwise from firing at the Soviets unless they crossed into Finland with a large force, a restriction that the Germans adhered to.
Soviet intel had wrongly estimated that there were large German and Finnish air concentrations in Southern Finland preparing to bomb Leningrad, which the VVS sought to destroy by striking them first. In reality there were no German aircraft based in Southern Finland at the time, nor was the FAF preparing to bomb Leningrad. Additionally, especially on the first day of the operation on 25 June, the VVS pilots couldn't find Finnish airfields and airbases, so they went for their alternate targets which were usually situated in population centres.
Finland by 25 June was going to enter the war anyway. Without the Soviet attack probably a few days, perhaps a week later. The Soviet attack saved the Finnish government face from having to do so under more vague justifications (valid as they too may have been; after all the country during the Interim Peace of 1940-41 was stuck between a rock and a hard place). The Finns began offensive operations on the ground at the end of June, mostly in July and August, and quickly moved to not only retake almost all of the land lost in the 1940 Treaty of Moscow, but also moved into more defensively favourable terrain in East Karelia and on the Karelian Isthmus, where they halted their offensive and dug in for the next two-and-a-half years (although the period was by no means entirely static, with Soviet counter-offensives in 1942 and battles with Soviet deep-recon formations, many air battles, etc., but I digress).