Unlike World War I, where battles consisted of long lines of infantry advancing, with tanks here and there to help out, World War II combat looked very much like what we see today in Iraq. A bunch of infantry moving through an urban area, with a tank or two to help out. This sort of thing required a lot of practice to be really effective. Also, reconnaissance battalions were organized to contain both tanks and infantry, and they worked very well. It was suggested that this be done with mechanized infantry and tank. No, cant be done. Tradition!
As the decades rolled by, there were more and more examples of how infantry and tanks really operated, again indicating that the tanks and infantry should belong to the same battalion. That way they could more easily train together, in the same way that they would eventually fight. The U.S. Army kept trying to get their own senior commanders to accept the idea of combat battalions containing infantry companies and tank companies. Until the very end of the 20th century, these attempts were unsuccessful. Now the army has incorporated the combined tank/infantry battalions in its latest reorganization. Each Heavy Brigade will have two Combined Arms battalions (each with two tank and two mechanized infantry companies.)
As was again demonstrated in Iraq, tanks can rarely operate without some infantry. In Iraq, you typically find one to three tanks operating with each platoon of infantry. The number depended on the situation. But, as in the past, the tankers and infantrymen had to hustle to catch up with the training required to make the tank/infantry team work with peak efficiency. With the new organization, that army brigades are going through now, and wont complete until the end of the decade, such catch up training will be a thing of the past.
Once more, the U.S. Army is trying to organize their mechanized infantry (that travel around in M-2 Infantry Fighting Vehicles) and tank units to train the way they fight. Why is that so much of a problem? Tradition! The first tank units were based on horse cavalry units. Naturally, the cavalry units were composed entirely of guys on horses. For equally obvious reasons, the infantry units were entirely infantry. But tanks were not just iron horses, they were designed from the start to operate closely with infantry. The first tank battalions went into action 90 years ago. By the time World War I ended in 1918, there were two kinds of tanks. One sort operated with infantry, giving the troops something to hide behind while enemy machine-guns were destroyed with the tanks guns. The other type of tank dashed ahead to get into the enemy rear area, there to destroy or disrupt support units. This was how traditional cavalry operated. Both types of tanks were organized into tank battalions, and it stayed that way in World War II. But in that war, things began to change.