NATO allies worry that the growing threat China poses may distract the US from Europe.A seapower expert argues the forces needed in Europe are different than in the Asia-Pacific.A confrontation over Taiwan would be primarily a naval fight; Europe would be a land battle.China's threat to Taiwan also poses a profound dilemma for NATO does it tie up its most powerful ally from European defense at a critical moment?A new study suggests that while splitting US military resources and focus is a problem, it's a manageable one. That's because the American forces needed to stop a Chinese fleet in the Strait of Taiwan are not the same as those suitable for destroying Russian tanks on the European steppe."For example, it is unclear whether armor and multiple launch rocket systems (MLRS) have as much utility in a Sino-American conflict a primarily maritime theatre as they do in Europe (although both capabilities have a role in each theatre)," wrote analyst Sidharth Kaushal in a report for the Royal United Services Institute.America has had to cope with the dichotomy between the Atlantic and the Pacific since World War II. In that war, the European theater was a ground conflict fought between huge mechanized armies and hordes of land-based aircraft, albeit with a smaller naval component in the sea battle to resupply Britain. The Pacific theater was a naval and air conflict fought among scattered islands, which put a premium on aircraft carriers, submarines and mobile amphibious units such as the US Marine Corps.Compounding the problem is that modern armies and weapons are expensive. The US could afford to fight a two-front war in 1941. Building a military sufficient to simultaneously defend Taiwan and Europe today would be ruinous, one of the many reasons the US maintains strong alliances in both regions. To be sure, the US backs Taiwan's independent government, but it's unclear whether it would commit forces to Taiwan's defense.However, Kaushal believes that the problem may be manageable by focusing on a couple of key variables. In particular, the success of an invasion of Taiwan depends upon China's ability to achieve sufficient air and naval superiority to protect the amphibious fleet ferrying assault troops and the logistics they depend upon."The crux of any defense of Taiwan will be denying it this superiority," Kaushal argued. "If this is achieved, it is likely that Taiwan's forces can mount a successful forward defense of likely landing sites on the island, making a protracted defense and the subsequent flow of US forces superfluous. If not, the battle will likely end before either of these things can be considered."In turn, this means that only some American military capabilities would be relevant, especially anti-ship missiles, submarines and mines. Naval drones, like those wielded by Ukraine, may also prove powerful weapons to attack a Chinese armada. Conversely, heavy armor and the capacity to transport it would be paramount in supporting NATO against a Russian attack.But sinking a Chinese fleet will require SEAD (suppression of enemy air defenses) missions against defensive systems protecting the invasion convoys. And it's there that the difference between the Atlantic and Pacific becomes manifest. US attack submarines are more needed in the Pacific than to defeat enemy armies in Europe, a seapower expert argues. MC1 Keenan Daniels/US Navy In Europe, Russian ground forces would be protected by land-based surface-to-air missile (SAM) batteries and radars, which are mounted or transported by truck. NATO would first have to locate them and then strike them with air-launched anti-radar missiles such as the US AGM-88 HARM, or ground fire from artillery, tactical missiles, and drones.A Chinese invasion fleet in the narrow 110-mile-wide Taiwan Strait, by contrast, would be an easily detected target and would be out of range of land-based SAMs on the Chinese mainland. Air defense would have to come from weapons on the Chinese warships, which can put out an impressive amount of firepower against hostile aircraft and missiles. China's Type 055 Renhai-class cruisers, for example, are equipped with 112 vertical launch tubes that can fire anti-aircraft or anti-ship missiles. The ships are also likely to be defended by fighter jets, many launched from airbases in China."SEAD in the European context is primarily a function of the ability to suppress or destroy a layered network of elusive and mobile ground-based SAM systems," Kaushal wrote. In the Taiwan Strait, "the challenge is not that targets are elusive, but rather that they are exceedingly well defended."SEAD weapons, such as the HARM missile, aren't really designed to hit ships. So rather than targeting Chinese air defenses specifically, the US and its allies should simply concentrate on sinking warships. Every Chinese warship destroyed is one less escort for the troop ships.In a Taiwan war, "SEAD is largely synonymous with anti-surface warfare and OCA [offensive counter-air]," Kaushal wrote. "If the picket of destroyers and cruisers protecting an invasion force suffers substantial losses and if [People's Liberation Army] aircraft, including AWACS, cannot freely operate over the Strait, the amphibious vessels on which an invasion defends are much less defensible." Advanced warning and control aircraft are flying radars that detect hostile threats for a wider force.A similar dichotomy between Europe and the Pacific applies to weapons needed for missile defense. "While all US IAMD [integrated air and missile defense] will be missed, some capabilities will leave more of a gap than others," Kaushal told Business Insider.In Europe, the threat is largely from Russian cruise missiles and short-range ballistic missiles. Against these weapons, the best defense is blast-fragmentation interceptors which detonate an explosive charge near the target or shorter-range hit-to-kill rockets (which kinetically slam into a missile) such as the US PAC-3 launched from the Patriot air defense system. In the Pacific, the threat is from Chinese intermediate- and medium-range ballistic missiles, such as the DF-21 "carrier killer" missile. Defense against them favors longer-range, high-altitude hit-to-kill interceptors such as the American SM-3 and THAAD.Weapons won't be the only precious commodity split between East and West. Running two regional wars comes with a problem beyond firepower: brainpower. It demands the US has enough decision-makers and staff officers to focus on their own theater, but distractions are common and even likely similar to the US loss of focus on Afghanistan after the 2003 invasion of Iraq. In World War II, the European theater had priority. Today, it's more likely to be the Pacific.Either way, Europe needs to build up its capabilities in case US resources are diverted to the Pacific. This includes air and missile defenses, hardened airbases, and improved anti-submarine warfare systems, Kaushal said.Europe accepts that America's attention will inevitably switch to the Pacific. "The prospect of the US needing to reallocate resources is increasingly viewed as a structural reality rather than a question of any given administration's policy," Kaushal said. But public discussion "tends to stop at the point where it is assumed that US resources will be stretched and that Europe must fill the gaps, with little granularity as to which resources and what gaps are of greatest concern."Michael Peck is a defense writer whose work has appeared in Forbes, Defense News, Foreign Policy magazine, and other publications. He holds an MA in political science from Rutgers Univ. Follow him on Twitter and LinkedIn.