Mortars have existed in some form or another for centuries, originally a simple "cup" filled with explosives and a projectile. This original form of the mortar resembled a mortar and pestle, from which the mortar derived it's name.
 
Although some semi-portable mortars where used in the late 1600's and American civil war, the British Stokes Mortar of WWI redefined artillery support. For the first time a single soldier could transport a mortar, and this mobility allowed mortars to complete many specialized jobs; often more accurately than the large artillery guns.
 
In Axis and Allies RTS the mortar regiment is usually deployed by Japan and Germany. Japanese mortar units have the best bombard and morale damage, and the German mortar regiment has 2 mg units, where all others have 1.
 
The mortar regiment's ability to inflict morale damage is it's main asset, wearing down the enemy and breaking their will to fight. Since no units have resistances to morale damage. mortars can be effective into the mech stage of the game; if kept in benefitial terrain and micro'd efficiently.
 

The basic build will vary some depending on map size and starting money (if more than $500)

If you are planning an early game Mortar attack, or your base is located in the front, you might spend more on regiments and less on the HQ's (which bolster your economy). If you have plans on really getting your economy going, or your base is located in the back, you might spend more on HQ's and less on your armed forces. Of course this is an example of a multiplayer build (3v3, 4v4), if you attempt to eco this much in a 1v1 you would be quickly rolled...

Get a basic idea of which upgrades effect which infantry units, and how to apply those upgrades in online play. Included is a suggested upgrade order to help newer players understand which upgrades are the most benefitial to their cause. See: Upgrading Your Infantry/Airborne

There are two different types of formations, formation on the regiment level (Assault, Aggressive, and Column), and formations on the division level, which are a formation of a group of regiments.

Groups of regiments can also be put in the Assault, Aggressive, and Column formation by activating group behavior, but this is not recommended for combat, only for reorganizing your division if it becomes too spread out. That means most of the time your division should have custom formations with group behavior set to off. This also allows you to set the formation for each regiment in your division without affecting the group formation.

You will want to use different group formations for different game situations, some basic examples are seen on the left and right. The determining factors are usually what type of enemy regiments you are facing, and how many special op bombs your opponents have. Hotkeys are essential for effectively managing your divisions, so once you have selected and organized a group, save it by holding down [Ctrl] and pressing any of the keyboard number keys; or simply right click an spot on the group control bar.
 
Of course there are many other factors that influence a successful infantry campaign, see conclusion for more tips and links.

Bombardment is an attack by artillery fire directed against fortifications, troops, towns, and/or buildings. It allows your forces to stand-off and attack the enemy from a distance, the benefits of which are obvious.
 
Bombarding can break your opponents will to fight, and effect the moral of targeted regiments, which can be key in determining who wins and loses any given battle. Bombarding with mortars obviously happens on a smaller scale than with arty mech or arty tanks, but the same principals still apply.
 
One method for covering your troops with artillery is to use the guard command. Select a group of mortars and tell them to guard a friendly regiment on the front lines. As long as they have healthy mortar units they will stay behind the guarded regiment and bombard anyone who attacks them. This works for all type of artillery, just keep in mind that a regiment will not heal while bombarding.
 
Bombarding can be used to remove the zone of supply from an enemy city. To place a city under siege simply bombard the city center.

Can't get enough stats? Here's a breakdown of the Mortar regiment by unit, before/after upgrades. Click on the icons to sort the columns

Here's our unofficial ranks, by unit and regiment for mortar infantry. Click on the icons to sort the columns