The enemies won't hesitate to fight with you right from the start. They will typically appear as a group of three foot soldiers rushing you from the right, and one gun solider chasing you from the left. The first supply runner will appear off the ground level. It's a good idea to remove any existing enemy forces from the screen before attempting to defeat the supply runner and collect the flame thrower. Watch out for the grenade launcher at the end of the bridge. Either jump over it the grenade if you are running along the middle level, or run beneath it if you are running along the ground level.
The soldiers will continue their attack as you approach the trucks. The ground in front of the very first truck contains three land mines. You can either hop over each of them, climb up the ladder and run along the truck (watch out for the gunners that follow you), or if you have any flame thrower ammunition, you can squat on the ground and fire at the mines to remove them. Don't fire too closely though, or you'll get hit by the explosion. It is possible to jump the gap between trucks, but not entirely safer to do so, especially if you miss and land on the ground in front of, or on top of, an enemy soldier. After the trucks stop appearing, keep running straight through enemies until the scrolling stops.
When the last gunner and foot soldier are removed from the screen, a truck will rush across from the left to the right (you don't have to fear getting run over.) When it stops, three waves of soldiers will pour out of the back of the truck to attack you. Each wave will consist of a group of foot soldiers who run at varying speeds, followed by one combat specialist who will jump kick you. If you have three flame thrower rounds, you can stand at the very left edge of the screen and hold your fire until a foot soldier gets close enough to attack you. Then you can fire off a jet of flame to remove the entire wave, including the combat specialist. If you run out of ammo, simply hold your ground and stab in front of you as fast as you can, and you should have no problem surviving.
NES differences
No gun soldiers appear immediately on the first level, only foot soldiers. In the Japanese version, where you must accumulate special weapons, the first supply runners are typically found waiting out on the highest level of the bridge, not the middle level. The set of mines occur before the first truck. Combat specialists typically first appear running on top of the truck so as long as you stay on the ground, you'll avoid them. There's one extra land mine on the way to the end of the stage. The makeup of the soldier that attack you in the final combat have been reversed, comprising a set of combat specialists who will all jump kick you followed by one foot solider. The large number of soldiers appearing along the floor will make the sprites flicker quite a bit on the screen.