In 1905, aged seventeen, Canaris joined the German Imperial Navy and by the outbreak of World War I was serving on board the SMS Dresden as intelligence officer. This cruiser was the only ship that managed to evade the British Fleet for a prolonged period during the Battle of the Falkland Islands in December 1914, largely due to his excellent deception tactics. Whilst anchored in Cumberland Bay, the Dresden was trapped and forced to scuttle by the British. Most of the crew became prisoners in Chile in March 1915, but Canaris escaped in August 1915, using his fluency in Spanish; with the aid of some German merchants he returned to Germany.
He was then given intelligence work and sent to Spain, where he survived a British assassination attempt. Returning to active service, he ended the war as a celebrated U-boat commander in the Mediterranean, credited with eighteen sinkings. By that time he was an enemy on the top of the list of the MI6 secret service in England. He spoke English fluently (as well as four other foreign languages) and as a naval officer of the old school, he respected the British Navy despite the rivalry between the two nations.
In 1919 Canaris married Erika Waag, also the child of an industrialist. They had two daughters, Eva and Brigitte.
After Adolf Hitler's rise to power in 1933, Canaris was made head of the Abwehr, Germany's official military intelligence agency, on January 1 1935. Later that year, he was promoted to Rear Admiral. During the period 1935-36, he made contacts in Spain to organize a German spy network there. His excellent Spanish made him the very man for the job. He was the moving force behind the decision that sided Germany with Francisco Franco during the Spanish Civil War, despite Hitler's initial hesitation to get involved in such an adventure. In 1937 he was still a supporter of Hitler, considering him to be the only solution against communism and a hope for the national revival of Germany. By 1938, however, he had realised that Hitler's policies and plans would lead Germany to disaster and secretly began to work against the régime. His personal style as a gentleman was incompatible with the thuggish behaviour of most of the Nazi party members. A letter from a Spanish contact of his has been preserved and unambiguously confirms his opposition to the Nazi regime. He tried to hinder Hitler's attempts to absorb Czechoslovakia and advised Franco not to permit German passage through Spain for the purposes of capturing Gibraltar. According to written sources, all of Franco’s arguments on this stance were studied and dictated in detail by Canaris, and that simultaneously, an important sum of money had been deposited by the British on Swiss accounts for Franco and his generals to further convince them to be neutral.
He also became involved in two abortive plots to assassinate Hitler, first in 1938 and again in 1939. His most audacious attempt was in planning, with Ewald von Kleist-Schmenzin, to capture and eliminate Hitler and the entire Nazi party before the invasion of Austria. At this particular moment, von Kleist visited England secretly and discussed the situation with British MI6 and some high ranking politicians. There, the name of Canaris became widely known as the executive hand of von Kleist in the event of an anti-Nazi plot. The high ranking German military leaders believed that if Hitler invaded Austria, or any other country, then England would declare war on Germany. MI6 was of the same opinion. However, the British reaction to the eventual Austrian Anschluss (and Hitler's annexation of the Sudetenland) was more cautious. At a meeting with Hitler in Munich, British Prime Minister Neville Chamberlain chose diplomacy over war. Munich was a severe disappointment for Kleist and Canaris. It gave Hitler's international reputation an important boost for two reasons: one, he was able to play the part of a man of reason and compromise; and two, he could boast that his predictions that England would not respond with war had proven to be correct. There are claims that Canaris, who was extremely shocked by this 'dishonest and stupid decision' (his own words), decided to be cautious and wait for a better time to act against Hitler. Nevertheless, it appears likely that MI6 maintained contact with Canaris even after the Munich Agreement. When Winston Churchill came to power, Canaris' hopes were renewed, given the new Prime Minister's strong position against Hitler.
Shocked by these incidents, Canaris began working more actively, at increasing risk, to overthrow Hitler's régime, although he cooperated with the SD to create a decoy. This made it possible for him to pose as a trusted man for some time. He was promoted to full Admiral in January 1940. With his subordinate Erwin Lahousen, he formed a circle of like-minded Wehrmacht officers, many of whom would be executed or forced to commit suicide after the failure of the July 20 Plot. It has been speculated that there was contact with British intelligence during this time, despite the war between the two countries. It is thought that during the invasion of Russia, Canaris received a detailed report of all the enemy positions that was known only to the British. The head of MI6, Stewart Menzies, who shared Canaris’s strong anti-communist beliefs, praised Canaris’s courage and bravery at the end of the war. Reinhard Heydrich and Heinrich Himmler, however, investigated in detail the sources of Canaris's information on Operation Barbarossa, arriving at the conclusion that there had indeed been contact between him and the British. After 1942, Canaris visited Spain frequently and was probably in contact with British agents from Gibraltar. In 1943, while in occupied France, Canaris is said to have made contact with British agents: he wanted to know the terms for peace if Germany got rid of Hitler. Churchill's reply, sent to him two weeks later, was simple: "Unconditional surrender".
During Heydrich's posting in Prague, a serious incident put him and Canaris in open conflict. A British agent — the Czech Paul Thümmel — was arrested by Heydrich, but Canaris intervened to save him, claiming he was a double agent actually working for Abwehr. Heydrich suspected that Thümmel was actually Canaris's MI6 contact. Heydrich requested that Canaris put the Abwehr under SD and SS control. Canaris appeared to retreat and handled the situation diplomatically, but there was no immediate effect on the Abwehr for the time being. In fact, Canaris had established another two links with MI6 — one via Zurich, and the other via Spain and Gibraltar. It is also possible that Vatican contacts provided a third route to his British counterparts.
Canaris also intervened to save a number of victims of Nazi persecution, including hundreds of Jews. Many such people were given token training as Abwehr "agents" and then issued papers allowing them to leave Germany. One notable person he is said to have assisted was the then Lubavitcher Rebbe in Warsaw, Rabbi Yosef Yitzchok Schneersohn. The assassination of Reinhard Heydrich in Prague, organized by MI6, was done in part to preserve Canaris in his important position.
The evidence that he was playing a double game grew, and at the insistence of Heinrich Himmler, who had suspected him for a long time, Hitler dismissed Canaris from the Abwehr in February 1944, replacing him with Walter Schellenberg and merging most of the Abwehr with the Sicherheitsdienst (SD). Some weeks later, Canaris was put under house arrest, preventing him from taking part directly in the July 20 Plot to assassinate Hitler. However, just after the Stalingrad disaster, Canaris had already planned a 'coup' against the entire Nazi regime in which many Nazi officials would be accused for known crimes, while Hitler would be arrested as an insane person based on his exposure to poison gas in World War I, then imprisoned for life. After the July 20 Plot, Canaris's long-time rival, SS leader Heinrich Himmler discovered that one of the officers involved in the plot, a friend of Canaris' who committed suicide, had kept the plot details in a metal box. The investigations also revealed that a number of other assassination plots (possibly another 10 or 15) had been activated but had failed and were covered up at the last minute. Most people who participated in these plots were people Canaris knew well. Himmler kept Canaris alive for some time because he planned to use him secretly as a future contact with the British in order to come to an agreement to end the war with himself as the leader of Germany. Hitler also wanted to keep him alive in order to get the names of additional conspirators. When Himmler's plan failed to materialize, he received the approval of Hitler to send Canaris to an SS drumhead court-martial that sentenced him to death.
Together with his deputy General Hans Oster, military jurist General Karl Sack, theologian Rev. Dietrich Bonhoeffer and Ludwig Gehre, Canaris was humiliated before witnesses and then executed on April 9 1945 in the Flossenbürg concentration camp, a few weeks before the end of the war. At the time of his execution, Canaris had been decorated with the Iron Cross First and Second Class, the Silver German Cross, the Cross of Honor and the Wehrmacht's Twelve and Twenty-Five Year Long-Service Ribbons.
Erwin Lahousen and Hans Bernd Gisevius, two of Canaris' main subordinates, survived the war and testified during the Nuremberg Trials about Canaris' courage in opposing Hitler. Lahousen recalled a conversation between Canaris and General Wilhelm Keitel in which Canaris warned Keitel that the German military would be held responsible for the atrocities in Poland. Keitel responded that they had been ordered by Hitler. Keitel, who also survived the war, was found guilty of war crimes at Nuremberg and hanged.