It is an article of faith that Australia was wide open to Japanese invasion in early/mid 1942, that Australia was saved by US forces in the battle of the Coral Sea.

The only problem with this scenario is that it is unsupported by the facts.

The first point to remember is that China was Japans military goal. the war in the South Pacific was a diversion that was forced on Japan by a lack of available oil and the imposition of an oil embargo.

The second point to remember is that Japans South Pacific strategy was NOT to militarily defeat the USA, they knew that was beyond their capacity, they intended to defeat the US Pacific fleet to buy time for them to sieze a ring of islands and fortify them against attack, presented with such a fait accompli the US was expected to negotiate a peace settlement rather than fight.

New Guinea was one of the places to be held and fortified. Australia was not.

A line from Malaya, Singapore, through the DEI, New Guinea and up along Islands to the East of Japan was to be held, once the islands were fortified, it was considered that Japan and it's internal supply lines would be secure and the main fight, China, could be safely resumed.

Why attack the USA at all? Because US forces (air and naval) based in the Phillipines, could interrupt Japanese fuel convoys in the South China Sea in from Borneo oilfields and the Dutch East Indies at will.

Shipping

 December 1941, Japan had almost 6.5 million tons of shipping, however the Japanese economy and existing military needs required 6.5 million tons of shipping to operate, there was no leeway and any diversion of shipping had to be temporary as it would mean cutting into strategic materials stockpiles. (The Japanese Merchant Marine in WW2 by Mark P Parillo).

In December 1941, the Japanese tanker fleet totalled only 575,000 tons. (Brute Force by John Ellis).

The IJA drafted 2,160,500 tons of shipping (roughly one third of the fleet!) at the start of the Pacific war (for supply and invasion forces), the IJA estimated that after July1942 they would be able to return all but 1 million tons.

The Imperial Japanese Navy (IJN) drafted 1,740,200 tons of shipping- (270,000 tons of which was comprised of tankers, almost half of the available tanker fleet).

All up this was roughly 2/3 of the available shipping diverted to military control, leaving the economy with only 1/3 of the minimum shipping it required to function. But it is worse than that ? 840,000 tons of the shipping left under civilian control were passenger vessels ill suited to cargo transport. (The Japanese Merchant Marine in WW2 by Mark P Parillo).

The practical effect of this is that, even without US intervention, Japan was not in a position to keep even this amount of shipping under military control in an effort to invade Australia, such an effort would destroy their economy.

The opinions of the major players.

29 Jan 42 Yamamto was only prepared to commit to invasions of Lae, Salamoa, Tulagi and Port Moresby. He was not interested in an invasion of Australia, that was a waste of manpower and too roundabout a way to end the war speedily.

(BBS, Daihon?ei kaigunbu, rengokaitai, Vol 2, p.309; and Nanto homen kaigun sakusen, Vol 1 p.355).

7 Mar 42 at the Combined IJA and IJN HQ liaison conference mid level naval officers put forward a plan to invade Australia, it was ridiculed by the IJA leadership as ?gibberish? and it was noted that troops were not available and logistic problems were quoted.

The IJN leadership (the direct superiors of the officers proposing the invasion) left it at ?shipping is not available?.

From that conference emerged the ?Fundamental Outline of Recommendations for Future War Leadership? paper, signed by the Combined IJN and IJA HQs at the liaison conference and presented by Tojo, Sugiyama and Nagano to the emperor on 13 Mar 42 states in Paragraph 3, as a ?future option to demonstrate positive warfare? that ?a temporary invasion of port Darwin , if and when the situation allowed; that is, if Chaing Kai Shek could be brought down and the Soviet threat removed?

(BBS, Daihon?ei kaigunbu, rengokantai, Vol 2, pp 337-8).

To put it simply ? the US is not mentioned by the Japanese as a factor and in their eyes it would only take the defeat of China and the USSR to make a limited invasion of Australia feasible ? all of this at the height of Japanese 'victory disease' and well before Coral Sea, let alone Midway.

The 'Plan'

The only plan advanced for an invasion of Australia was that advanced by mid level naval officers on 7 Mar 42, it was ridiculed (as noted above) and it was noted that troops were not available and logistic problems were quoted by both the IJA and IJN leadership.

It is important to note that the IJA were the only Japanese force that had any large scale operational experience against Commonwealth troops on land and they estimated that it would require 12 Divisions to make a limited invasion of Northern Australia (they were also very clear on the point that they did not have 12 Divs to spare), they also noted 'logistic problems' which was quite an understatement.

The defending forces

Frankly, even if it had been possible to deliver Japanese 12 Divisions to Northern Australia, they would have been quickly defeated. in offensive operations a ratio of 3 attackers to 1 defender is assumed as a prerequisite for success, but that is dependent on the attacker having equipment parity with the defender, the Japanese were poorly equipped for open country warfare, Australian produced 2 Pdr anti tank guns could penetrate Japanese tanks at any range they could hit them, an Australian Division had a 'throw weight' of supporting artillery that was twice that of a Japanese division and threw their shells further than the Japanese guns.

Australia, in 1942, was able to almost fully motorise 8 Divisions (and had 8 Divisions worth of troops in Australia at the time) and was also in a position to requisition vehicles as neccessary from the large number of available civilian vehicles to make up any shortfalls.

The Australian army status report (AA MP729, series 6, file 42/401/142) stated that 22.5% of all major units were ready for mobile offensive ops at Bde or higher level; 48% were ready for static or semi static ops at Bde or higher level; 81% of all major units were ready for static or mobile ops at Bn level. 19% had not completed Bn level training.

So the majority of the force available (81%) was trained to a usable standard, the rest ? like many military forces in history, would need to learn on the job, but would likely be held back from the most demanding of tasks whilst that training was completed.

Australian produced small arms (Bren guns, Vickers HMG, Lee Enfield .303 rifles, Owen Guns) and ammunition were certainly a match for the best the Japanese forces had.

The only area of superiority the Japanese had was in mortars, particularly light mortars, which are a lot more useful in jungle than they are in open country, however Japanese medium mortars also outranged the 3 inch mortars produced in Australia- this advantage would of course be offset by the Australian divisional artillery advantage mentioned above.

By 26 June 1942 (mid 42 is the earliest 'credible' date for a Japanese invasion, factoring in troop transport, resupply, refitting etc) Australia had taken delivery of 103 Matilda tanks and by the end of April 1942, 54 x M3 Mediums (Lee/Grant) had arrived in country - by December 1942 a total of 757 x M3 Mediums were on hand, both of these tanks were virtually impervious to all Japanese AT guns available at the time and were armed with a gun that could penetrate the armour of any available Japanese tank at the time at virtually any range.

10 x M3 Light tanks (Stuart) had been delivered by the end of 1941. Deliveries increased during 1942 and by the middle of 1943, and the total number of Stuarts in Australia was around 370 tanks ? the Stuarts gun was as effective as the 2 Pdr, but being a light tank, it was more vulnerable to Japanese tanks and AT guns than the Grant and the Matilda.

Australia also produced Bren gun carriers, including variants that mounted 2 Pdr AT guns and 3 inch Mortar variants.

Basically, Australia had better artillery, better AT guns, better tanks and had an ability to fully motorise the force available, meaning they could dictate the time and place of an attack and could break contact virtually at will. As well as having the huge advantage of much shorter supply lines.

Churchill had also promised that he would divert a British Armoured Division from the Middle East to Australia in the case of 'substantial invasion'.

Naval Power

The IJN had no force capable of blockading Australia, as too many Australian ports were way too far south and the IJN was not good at replenishment at sea, nor did they have enough tankers to support such a plan.

Any attempt to use IJN forces for shore bombardment would require diverting a signficant portion of the Japanese fighter a/c to protect it from air attack.

Shore bombardment operations down the east (or west) coast of Australia with significant fleet units would also put an intolerable strain on both fuel oil supplies and the tankers that would need to move such supplies.

To get a feel for just how much fuel this would consume, the excellent Combined Fleet website link has detailed information on the fuel consumption for a Japanese bombardment task force in the Solomons.

Airpower

  Japan certainly had an advantage in this area, but the Japanese air forces (Army and Navy) achieved most of their successes by attacking their enemy on the ground, this option was denied them as, by June 1942, there was not only an extensive ground observer network but also a widespread radar network in eastern Australia.

This would largely allow the Australian force to choose its battles and only fight when the odds were in their favor, added to this is the large numbers of heavy and medium AA guns that the Australian army possesed, the abundance of machine guns in Australian army units along with the light construction, lack of armour and lack of self sealing fuel tanks on Japanese aircraft and Japanese airpower could not be considered a decisive factor, particularly when constrained by such long supply lines.

An example is China where Japanese airpower was virtually ineffective in supporting the battlefield in spite of their total air superiority and the paucity of Chinese air defences.

Australia had 180 front line fighters (mostly Kittyhawks), as well as 315 medium bombers and aprox 1000 light bombers, these aircraft, if used in conjunction with the observer network and the radar network to attack undefended or lightly defended targets could not only inflict significant damage on a force (such as the Japanese) that was very lightly equipped in anti-air weapons, but could force the Japanese fighter units into a mostly defensive role.

Conclusions

The Japanese were unable to invade Australia largely for logistic reasons, but those logistics were greatly complicated by the Australian governments development of an fairly sizeable arms industry, that made Australia largely independent in most categories of ammunition and most critical weapons types. This, effectively 'raised the bar' for what would constitute a credible invasion force well beyond that which the Japanese could deliver and supply.

The Allies knew the Japs were not coming as of mid April 1942 as noted in the Allied Combined Operations Intelligence Centre minute, 11 Apr 1942, [NAA] MP1587;218s SRs 575 ?Magic summary?, 18 April 1942, RG257, US National Archives and Records Administration.

Some excellent references that relate to the subject-

"Armed and Ready" by AT Ross.

"The Japanese Merchant Marine in World War 2" by Mark P Parillo

"Japan's Southward Advance and Australia" by Henry P Frei

"Brute Force" by John Ellis

Excellent read Kevin PORK.

Getting back to the current situation, in The Australian today is the following quote from retiring Chief of Army Geneal Peter Leahy.

"He said he could also see the possible deployment of the army's Abrams tanks as well as heavy artillery to Oruzgan province in Afghanistan in the event that existing capabilities provided by the Dutch were withdrawn.

"I think that is one of the potential deployments for not only the Abrams but artillery. Currently in the south of Afghanistan the Canadians have Leopard tanks.