DKM GRAF SPEE´s armour concept is alongside ideas formulated in the interwar period with A/N schemes beeing considered to be important. She is well armoured and protected for a cruiser of her period, while many contemporary cruisers lack either armour thickness or coverage.
The idea behind her concept was to keep shells out of the ship´s vitals or- in case of the embedded vitals- if that´s not possible to detonate them in the main bulwark space with the major longitudinal armoured torpedo bulkhead behind acting as a splinter catcher thus providing additional protection to the vitals.
Most contemporary cruisers had insufficiant armour protection in the three main areas of protection:
(A) underwater protection
(B) protection of the embedded vitals (ship controll, magazines and machinery spaces)
(C) protection of the exposed vitals (comm tubes, conning towers, firetops, barbettes and turrets)
[A] underwater protection:
Except for the longitudinal major seperating bulkhead provided to the japanese heavy cruisers starting with MOGAMI, no other cruiser employed a full blown anti torpedo defense consisting of longitudinal armoured bulkheads. In DEUTSCHLAND, these were made from Nickel Steel armour and couldn´t be put down to the ships bottom owing to weight considerations (and this in fact created a zone of weakness) and terminated at the joint with the double bottom. The system was unsound against torpedoes but it was also designed to augment the rather light 50-80mm thick main armour belt and when beeing torpedoed by HMS SPEARFISH in april 1940, the whole stern collapsed and she shipped some 1,300ts of water. The torpedo hit outside the TDS but it remains unlikely that such a massive blast could be contained by the narrow TDS amidships. It was tested in june 1941 when she suffered another torpedohit by a Bristol Beaufort. Despite the rather light warhead of this torpedo, the TB bulged in (as designed) but the lower joint failed echoing the design weakness of the bulkhead mentioned above, allowing much water to flood the port machinery spaces.
It may be questioned whether or not a full blown TDS is benefitial on a cruiser sized ship like DEUTSCHLAND, generally but the RMA sticked to the idea. The next ship SCHEER had a slightly thinner bulkhead (40mm) made from a new and extra ductile new Ww material, but still running not down to the bottom of the ship. Only GRAF SPEE had a 45mm Ww bulkhead reaching from the main armour deck down to the bottom of the ship (inclined 15 deg). I already mentioned that inclined bulkheads tended to fail on real life impacts (which is why RN, the DKM, the FN and the USN abandoned them in their last designs).
Nevertheless it represented some kind of protection not aviable to any cruiser at her own time. Effectively, the armoured bulkhead also catched diving projectiles (only japanese cruisers had main belts which in theory could do this, running down highly tapared, to the ships bottom).
protection of the embedded vitals
Main belt protection of GRAF SPEE was strong. The 100mm thick belt extended for a height of two decks or 4.7m (15.5ft) and covered the whole citadell externally while beeing inclined 12 deg. In addition there were splinterproof bulkheads running both, behind (TDS) and in top of the belt (40mm thick for two deck heights).
In contrast to this, the side belt of other Treaty Cruisers varied between 3in and 4in (only locally at the magazines, sometimes not externally but internally like in the COUNTY´s), with the coverage beeing very small (in japanese cruisers the height coverage was aviable for less than 5ft instead of the more than 15ft in the GRAF SPEE, rendering any kind of belt hits unprobable in the first place).
The armour decks were 40mm (1.57in) thick over the machinery spaces and 70mm (2.75in) thick over the magazines, which compares favourably with period contemporaries which typically showed figures of in between 1.0in and 1.5in for the machinery spaces and 1.5in - 2.25in for the magazines respectively (PENSACOLA-class, NORTHHAMPTON-class, NEW ORLEANS-class, TRENTO-class, COUNTY class, TAKAO-class, MYOKO-class). Only the french ALGERIE and the italian ZARA-class cruisers approached or exceeded that level of protection in some areas (100-150mm main belt, 70-80mm over magazines, 25mm over machinery spaces) but both fell short in others. Japanese cruisers of this period typically had that level of thickness (4 to 5in belts) but the coverage was very low, often less than 5ft for max. belt thickness before taper began to reduce it and the deck armour was light (~30mm). There was no additional protection with internal splinterbelts provided to any of these designs except for ALGERIE.
[C] protection of the xposed vitals
The CT of GRAF SPEE was 150mm (5.91in) thick, which compares favourably against all but the newest CA designs (typically 3 to 4in except for the newest US designs which tended to have up to 7in armoured CT´s). Her Barbette´s were armoured 130 to 140mm thick (5 to 5.5in), a good level better than the 1 to 3in typically found in Treaty cruisers (with the exception of ZARA and the BALTIMORE-class) and while most other cruisers of her period had extremely thin turret protection (1in typically in japanese, british cruisers and old US cruisers, 3-4in in italian and french cruisers and 5in in the newest ZARA-class and BALTIMORE´s), GRAF SPEE´s main turrets were provided with 145mm (5.7in) to 170mm (6.7in) thick armour.
The most critical component in her fighting against cruisers is that it´s main battery could defeat any of them with HE rounds (typically able to defeat 1.75in US STS aequivalent of armour with lateral fragmentation in some 3 ft. distance and able to defeat up to 4in of armour with HE/common at contact, which puts all of the aforementioned designs at risk through turret and /or deck perforation), while a truly modern hard capped, small filler APC was necessary to deal with the very sound armour protection system of GRAF SPEE. These didn´t contributed to the effect behind the plate in the same way like HE did, for comparison: An 11.13in HE shipped some 48.06lbs of high explosive, while an 8in APC contained typically 3 to 4lbs of high explosive. The damage GRAF SPEE traded the few time it engaged with it´s main battery prooved that it wasn´t even encessary to hit them, one turret of HMS AJAX was rendered unservicable by splinters while beeing straddled by 11.1in HE and straddles or shorts ruptured the hull of HMS EXETER in many places. The decision to armour Treaty cruisers weight efficiently A/N was putting them at risk here. More armour applied to larger surfaces may have helped moren than a thick but eventually to narrow main belt and GS wouldn´t need to use APC against them, putting the soft or lightly armoured parts of the ships at risk of the nondelay HE or short delay HE/base fused. This allows the PBB to overpower a cruiser. It requires a well protected ZARA- or a BALTIMORE-class cruiser to force a PBB to shift entirely to HE/base fused or APC.