Here's an awesome answer by /u/hannahstohelit on Eichmann's capture in Argentnia and I have also written previously on the issue of Eichmann's capture:
So, officially, there was only one of these operations that are known to have succeeded (there might have been more and there were a couple of operations that didn't work out) and that therefore required an international reaction and consideration of legality. While Israel did indeed try a couple of other Nazi criminals, those cases were managed in cooperation with the country where they hid, like the US or Canada, and where because of false statements these criminals made at the time of their migration they could be deported to Israel. This famous case was the capture of Adolf Eichmann.
Adolf Eichmann, head of the Reich Security Main Office's Referat IV B 4, responsible for the organization of deportations of Jews to Auschwitz and elsewhere, had managed to flee to Argentina in 1950 after he had received false identity papers in the name of "Ricardo Klement" with the help of Bishop Alois Hudal and his Nazi-sympathizing collaborators in the Vatican. In subsequent years, he worked for Mercedes in Argentinia and send for his German family to join him in Buenos Aires.
During his time in hiding, he was still in contact with some of his former Nazi friends and from 1956 forward was interviewed by former SS-member Wilhelm Sassen, who wanted to write Eichmann's biography. In the tapes of these interviews Eichmann reveals that unlike his later self-presentation in front of the court in Jerusalem, he indeed was an ardent Nazi and anti-Semite and was of the opinion that the movement would rise again in Germany, giving him a chance to finish what he started.
In the meantime, Eichmann was recognized by several survivors in Argentinian, most importantly, German-Jewish emigrant Lothar Hermann who passed on the information that Eichmann was living there to German District Attorney Fritz Bauer. Bauer tried to instigate a German investigation and eventual capture of Eichmann but was met with strong institutional resistance by a German justice system unwilling to confront the possibility of a German Eichmann trial. Eventually Bauer was forced to at least do something and passed on the pertinent information to Mossad director Isser Harel, who had previously received similar information about Eichmann's whereabouts from Nazi hunter Simon Wiesenthal.
Harel dispatched Shin Bet chief interrogator Zvi Aharoni to Buenos Aires on 1 March 1960, who was able to confirm the suspects's identity as Adolf Eichmann. In light of Argentina's policy of providing refuge for Nazi war criminals, the Israeli government decided that Eichmann should rather be captured instead of asking the Argentinian government for extradition. A team under the leadership of Mossad operative Rafi Eitan was dispatched to capture Eichmann and bring him to Israel.
They captured Eichmann, held him in a safe house for nine days then spirited him on board of an El Al airplane and flew him to Israel where he arrived two days later, on May 22, 1960.
After it was announced to the world that Eichmann had been captured in Argentina and brought to Israel, the Argentinian government reacted not very positively. While there initial diplomatic approaches to Israel remained unsuccessful, the Argentinian government lodged an official protest with the UN in June 1960 for violation of their sovereignty. The Israeli representative at the UN at that point claimed that the capture of Eichmann had only been the act of a group of individuals acting on private initiative and therefore – while representing a violation of Argentinian law – the act did not constitute a violation of sovereignty.
The UN Security Council disagreed and on June 22, 1960 it adopted Resolution 138 in which it stated that Argentina's sovereignty had been violated and that Israel should pay reparations to Argentina. This was adopted with all votes and only Poland (then member of the Council) and the USSR abstaining from voting. Argentina and Israel eventually reached a mutual agreement to end the dispute on August 3, 1960.
So, while the UN security council had at the time the opinion that Israel had indeed breached international law, a variety of legal scholars have interpreted the situation differently: Nicholas N. Kittrie e.g. writing for the Journal of Criminal Law and Criminology under the title A Post-Mortem of the Eichmann Case in 1964 interpreted the situation differently.
Writing that it was indeed the case that Israel despite the lack of an extradition treaty could have negotiated Eichmann's extradition with Argentina – he was living there as an illegal alien under a false name after all –, this was highly unlikely given the internal political situation there. Emphasizing that Argentina and Israel issued a joint statement ending the dispute he writes:
However, after Israel made its apology the matter was declared closed in a joint communique issued by the two nations on August3, 1960. The failure of the United Nations and Argentina to insist upon Eichmann's return thus seemed technically to cure the illegality of the capture. As an individual Eichmann had no standing in international law, and whatever rights he possessed had to be enforced on his behalf by an aggrieved nation. The desire to avoid the embarrassing situation which the possession of Eichmann's person would have posed and the reluctance to awaken past memories kept the governments of Argentina (within whose boundaries he was found), Austria (of which he was a naturalized citizen), and Germany (whose Third Reich he served) from making a claim for or on behalf of Eichmann. Since all concerned nations waived their right to Eichmann, Israel appeared free, within the framework of its own law, to pursue with regard to him a conduct of its own choice.
What Kittrie is basically saying that without a government disputing these actions, the act itself becomes retroactively legal because in international law, the maxim of "when there's no plaintiff, there's no judge" is applied. He further argues that there is precedence found for similar cases all over English and US case law, especially pertaining to the seizure of ship's in international or another state's territorial waters and that the legality of the trial itself as well as the court's jurisdiction were not affected by this.
He concludes that most legal commentators agree on Israel's right to try the kidnapped Adolf Eichmann, if not by adherence to the standards of international law but by the general permissiveness of it, in that every state has the jurisdiction to to punish war criminals in its custody regardless of the nationality of the victim, the time it entered the war, or the place where the offense was committed, according the legal statutes created by the Nuremberg Trials.
In the end, the question of violation of international law became moot since Israel and Argentina settled the dispute surrounding Eichmann's capture thus retroactively giving the act if not legality then at least legitimacy.
Edited to add:
As reported by the JTA:
Armand Bernard of France told the Council he considered that the apologies for violation of sovereignty already given by “the highest authorities of Israel” provided Argentina with the “expression of satisfaction” she had sought.
A position later adopted by the Argentinian government in August 1960.
A claim has been also made that the Mossad was responsible for the assassination of Herberts Cukurs, also known as the "Butcher of Riga" in 1965. This claim originated from Gad Shimron's book The Execution of the Hangman of Riga, which was allegedly written together with the Mossad agent responsible for the assassination. In this case, however, the Mossad's involvement remains unclear and the archives have not been opened yet. Cukurs body was indeed found in the trunk of a car in 1965 together with several documents pertaining to his crimes committed in Riga after newspapers had received a note that it was there. Just what role the Mossad played in this is however still not entirely clear.
This lack of clarity pretty much suiting almost everybody involved, the assassination never drew much attention from governments or was tested for its legality under international law.
Sources aside those mentioned:
Bettina Stagneth: Eichmann vor Jerusalem.
L.C. Green: Legal Issues of the Eichmann Trial
Matthew Lippmann: The Trial of Adolf Eichmann and the Protection of Universal Human Rights under International Law