Human Military Forces

United by a common foe, the nations of Europe have joined forces in a historically unprecedented alliance. Signatories include Great Britain, France, Germany, Spain, Russia, America, and Italy. The alliance includes complete military and economic support between the member nations, however it has proved difficult to implement. The various members are so distant and have such varied strategic ideas that it is a major challenge for them to operate effectively, a factor which the Martians have capitalised on.

Infantry
A typical human infantryman will be armed with a bolt-action rifle with bayonet, and perhaps a couple of fragmentation stick grenades. They will usually be unarmoured. Human infantry is generally much better equipped and organised than standard Martian infantry, however they lack the firepower to complete with armoured units.
Infantry organisations (platoons, battalions etc.) vary considerably from nation to nation. Soldiers in any given theatre will be a mixture of different uniforms and equipment, each nominally under their own nations regional commander but in practise taking orders from the commander of the largest unit nearby.

Naval Forces
Before the Martians came the British navy had a 'two powers' system, where the Navy would also have at least as many ships as the next two largest navies combined. This system has served them well, since the British Navy stands unchallenged as the single most powerful force on Earth's oceans, especially with the assistance of the other human nations. Unfortunately this Naval supremacy isn't as dominating against the Martians as it historically has been against other human nations due to the Martian use of flying vehicles. Ironclads have some use against the low-flying ships of the common Martians, but can do nothing but watch as Overlord vehicles soar far overhead.

Armour
Inspired by the Martian Fighting Machines, British engineers created the first tanks in the early years of the war. Although designed to destroy the Fighting Machines, tanks have actually proved best used against infantry. The best armour man can make is useless against the heat-ray, and even the cannons carried by a tank are only marginally effective against Fighting Machines. However in the same way that human infantry are powerless against the Overlord vehicles, so too the Martian infantry have no weapons that can defeat a human tank.

Artillery
Human forces make extensive use of Artillery in defense because large cannons are the only reliable means of destroying Fighting Machines. Unfortunately large guns aren't usually mobile enough to be effective offensive weapons, but a city protected by a row of cannons is much safer. A few new designs of self-propelled guns have been created, which are basically just lightly armoured tanks with massive guns.
Artillery is usually breech-loading, firing solid, shot, shell. Cannons are rated as x-pounder, based on the weight of the projectile without its casing or powder. Some forces, notably in Asia and South America are still using muzzle-loading cannons.