Intelligence was mainly handled in Nazi Germany by two organisations. The Wehrmacht had their own military intelligence service called the Abwehr, which was commanded by Admiral Wilhelm Canaris. The SS also had their own intelligence service called the Sicherheitsdienst or SD for short. However, relationships between the two services were very cold due to their overlapping areas of operation and the fact that the Abwehr, as professional soldiers, felt that the SD was an amatuer organisation run by common thugs. Admiral Canaris had a very difficult relationship with Reinhard Heydrich, the head of the RHSA, under which the SD was placed. The two would often have arguments and this limited the ability of the two services to work together.

Matters were also not helped by the fact that Admiral Canaris, in conjunction with a number of senior Abwehr officers (Colonels Lahousen, Oster, Hansen, Pieckenbrock, von Bentivegni, Groscurth and Freytag-Loringhoven) were actively trying to undermine the Nazi regime. Colonel Hans Oster was a leading member of the wider military circle trying to kill Hitler, and detonators captured from English intelligence agents provided by Colonel Erwin Lahousen were used in at least three attempts to kill Hitler including the 20th of July plot. Canaris, Oster and would be executed for their part in the plot, while Freytag-Loringhoven killed himself before he could be arrested. The Abwehr were also hurt by the defection of 'Harlequin', a high ranking agent who provided the British with valuable information on Abwehr agents and practices. His true identity is still classified.

In terms of undermining the war effort, the Abwehr would regularly claim that their efforts were behind various ship sinkings or industrial sabotage in order to inflate their statistics when actually this was not the case. While sabotage attacks against British ships were carried out in foreign ports, these were not particularly effective and only had moderate results. Canaris avoided appointing committed Nazis to positions within the Abwehr and where possible would bring in officers who were sympathetic towards the cause. When ordered by Keitel to assassinate the French generals Giraud and Weygand, Canaris disregarded the orders and took no steps towards doing so. Canaris was involved in the planning for an invasion of Gibraltar and possible Spanish entry into the war, and while there is no direct evidence that he actively sabotaged the attempts to get Spain to enter into the war, he certainly didn't exert himself. Obviously, Spain stayed neutral. When the Abwehr recieved intelligence that the SD were planning to kill the Italian King or even the Pope, Canaris passed a warning to the head of Italian intelligence during a meeting in Venice. Colonel Oster also passed the date set for the invasion of France and the low countries to a contact he had in the Dutch embassy. However, since the date was changed so many times due to indecision in the High Command, it was believed that Oster was crying wolf, and so his warnings were disregarded.

However, the Abwehr were also involved in funding and training members of the IRA. One member of the IRA, Sean Russell, was smugled to Germany to be trained in the Abwehr sabotage school, although according to British intelligence he was already a skill saboteur so it's unlikely this would have been particularly useful. Any plans the Germans had to use the IRA were confounded after Russell died from unknown causes on his journey home to Ireland. The Abwehr also transferred funds to the IRA and made were in contact with Irish-Americans in the USA regarding sabotage operations there. A number of plans were set up to recruit intelligence networks and carry out acts of sabotage in the UK, but the effort was hindered by the arrest and internment of several agents by the Irish Government and others being forced to flee after their cover was blown. Sabotage agents parachuted into the UK were almost immediately captured and the Abwehr in occupied France only managed to send one agent to England, who returned soon after through Portugal, and was believed by Lahousen to be a double-agent. The lack of success in Western Europe was probably less down to incompetence on the part of the Abwehr and more down to the excellence of British counter-intelligence and an unwillingness to commit on the part of the leadership of the Abwehr.

However, the SD did have some notable intelligence successes. Both the SD and Abwehr were involved in the planning of Case Green, a staged attack on a German radio station and forestry hut on the Polish border in order to give an excuse to invade Poland. The attack was carried out remarkably professionally and concentration camp prisoners dressed in Polish uniforms were killed and scattered around the sites. However, its convincingness was probably undermined by the fact that Arthur Nebe, a senior SS official and member of the resistance, might have leaked the date set for the invasion to the British.

The other notable success by the SD was the so-called 'Venlo incident', where Walter Schellenberg, an SD agent, convinced two SOE agents in the Netherlands that he was representing a number of German Generals who wanted to discuss a coup against Hitler. It's not exactly clear if the initial contacts with the SOE were from disgruntled Generals and the SD later caught wind of it (this is the position taken by British Intelligence) or if the whole operation was an SD front to begin with (this seems to be implied by some of the German documents). In any case, the two agents, Major Best and Captain Stevens, agreed to meet with the 'General' near the German-Dutch border. When they arrived however, they were kidnapped by the SD and their liason with Dutch intelligence was shot and killed. This was a major win for the SD, and severely undermined the British intelligence network in the Netherlands. The British reportedly continued to try and make contact with the Generals for several weeks, broadcasting information that was very saluable to the SD.

Ultimately, the intelligence efforts of the Abwehr were undermined by the fact that their senior leadership was consistently anti-Nazi, and took steps to avoid actions against the Western Allies that they found to be distasteful or beneath them. Combined with the excellence of British counter-intelligence, this meant that Abwehr operations were, on the whole, completely unsuccessful. They failed to predict several major offensives including the invasions of Normandy and Sicily and had a reputation as being not particularly effective. I would personally say that this is not down to incompetence, as Canaris proved himself to be an effective intelligence officer earlier in his career, and his deputies demonstrated their intelligence capabilities by successfully running operations against the Nazi state for so long.

Sources:

'Interrogation of Erwin Lahousen', National Archives, WO208/4347

'File on Erwin/Arnim Lahousen, National Archives, KV2/173

'German Generals and the Venlo Incident', National Archives, FO1093/201 and FO1093/202

Canaris, the Life and Death of Hitler's Spymaster, Michael Mueller. (2017)

Order of the Death's Head, Heinz Hoehne. (1969)