Build Phase
Initially, you will start out with nothing more than a country and the resource points necessary to build an army.  The Build Phase is used to create and deploy your forces for the upcoming war.  Think of it as years of military planning and build up simulated in a single turn.
During the Build Phase, you can build, delete, and place units within your country’s borders without any kind of movement or financial penalty.  Position, reposition, build, and disband as many units as many times as you want.  You can also think of this as the “build now or pay later” phase because units built later cost twice as much, take up to five turns to complete training, and can only be built in cities.  See the “Building Units" and "Build Palette" sections for more details on creating units.

Short Cut:  use the "Build For Me" feature to have the computer generate an army for you.

Orders Phase
Once you leave the Build Phase, the game really begins.  The first Orders Phase will give you a cash starting bonus to spend as you like.  Depending on the Mobilization setup option you choose, you might get additional funds as well.  While you are in the Orders Phase, you issue orders to your troops, build reinforcements, review the last turn’s combat, and do everything else necessary to crush your opponents.  This is the “hands on” part of Onslaught.  When finished select "End Turn" from the "Turn" menu.

Resolution Phase
This is where all the action takes place.  Air strikes, bombardments, movement, ground combat, etc. will be resolved as you watch.  Everything that you and your opponents did during the Orders Phase will be carried out.  You can not interact with the resolution, but you can skip over it with a simple Command Period if you like.  When the Resolution Phase finishes you are dropped right back into the Orders Phase to figure out what to do next.

Air Movement and Combat

Air movement orders are the first orders carried out by the Resolution Phase.  Units will move according to their type, beginning with fighters, fighter bombers, medium and heavy bombers, and ending with air transports.  Patrolling enemy fighters and flak guns will try to attack moving air units.  When multiple interceptions or flak attacks occur in one hex they will be consolidated in a single report in the Combat Report palette .  Any damage and Condition loss your air units suffer from such attacks will be deducted from the unit’s statistics as they occur.
Bombardment

Artillery bombardments will be carried out in random order after air units have had their say.  Victims of Bombardment will receive any Strength and Condition losses as soon as they occur.
General Movement

Movement in Onslaught refers not to battle, but literally to units moving along the paths you have plotted for them.  After players are done issuing orders, the computer will randomly select a country and a unit within it, and then carry out that unit’s commands.  The computer will repeat this random process for every single unit on the map.  

A unit will move from hex to hex until it reaches its destination, runs out of movement, is blocked by friendly units, or bumps into an enemy unit.  If the latter happens, combat will occur but is carried out later (see below).
Ground Combat

If a unit can’t complete its movement orders because it ran into an enemy, the unit will mark the enemy hex for battle (if you move any other units in to the same hex they will join in the fighting).  All units attacking a contested hex will be grouped together as the Attacker and any enemy units in the hex will be grouped as the Defender.  All battles will be resolved in the order in which they arose, but only after all units on the map have had a chance to move.

When all ground attacks have been resolved the Resolution Phase is over and you enter the Orders Phase again.