McClellan was a naturally cautious general much in the vein of Bernard Montgomery (the British WWII Field Marshall). He was very much aware of the deficiencies of his own army. He was a lot less aware of the confederate problems. So he tended to want to cross every t and dot every i on every plan before entering battle.

To compound his natural caution, he had a frankly terrible intelligence service. He hired a bunch of spies from the Pinkerton service. They may have made decent private security, or police detectives, but they didn't have much in the way of military training. They tended to misreport the size of the Confederates, usually making them sound twice as large as they were. As far as McLellan was concerned he was consistently outnumbered.

Take Antietam. McLellan knew he had a bit under 90,000 men. He thought Lee had 100,000 or more, and so was reluctant to attack. When McLellan finally did attack Lee was able to use interior lines to move troops around and seemed to confirm that he had a lot more troops than the 40,000 or so he actually had. When it was over McLellan was worried about trying to attack Lee's as of yet unengaged reserves with his own tired men. That Lee had no such reserves was imcomprehensible to McLellan. In his mind he had attacked and miraculously driven off a larger force. In reality he ruined perhaps the best chance of annihilating Lee's much smaller army while they were pinned against the river.

With accurate intelligence, it's likely that he would have been a much better commander. He was consistently looking for a perfect chance to isolate and smash a Confederate army. I'm not sure he ever realized he actually had several such opportunities and let them all slip away.

Fast and slow are relative terms when it comes to warfare. Grant had a major numerical advantage over Lee outside of Petersburg, but that campaign lasted the better part of a year. Why was Grant so hesitant?

More directly, it's important to keep the specific circumstances of McClellan's operations in mind when assessing his decisions, as well as the broad trends in the course of the war. There was general consensus in the U.S. that the Union had to be preserved, but the question of what that meant and how it was to be achieved proved divisive indeed.

It was clear that slavery had been the issue that divided the country, but what did that mean in terms of action? Should the US explicitly accept and protect the institution, so as to remove the rebellion's raison d'etre, or should the US strike at the institution that divided the nation and punish the slavocrats responsible?

Was the Confederacy now a separate nation, and what does it mean if it is or isn't? If it's not, and the rebels are all just traitors, then that means Southern civilians are innocent until proven guilty, and that their property cannot be disturbed without due process of law. If it is a separate nation, though, Southern property can be seized under the laws of war, but that would mean a de facto recognition of the Union's division and of the Confederacy's right to exercise the prerogatives of a sovereign nation.

How much support does the Confederate government have from the Southern people? Are they just a bunch of selfish aristocrats who've hoodwinked a loyal population into the madness of secession, or is this a war of national liberation on their part, in which the Southern people are united in hostility?

These are tricky questions, and many people have allowed hindsight to warp their perspective on Civil War figures based on their answers to what they didn't even know were questions.

In McClellan's case, he came out of a political culture (Whig, then Stephen Douglas Democrat) that stressed compromise and Union that transcended sectional interests. Most Americans agreed that slavery was an institution particular to a given state, and that the national government had no right to interfere with it; given its importance in the states that had it, slavery was an essential part of the Union. Equally important to the Union was the stipulation that states retain a republican form of government; despotism was antithetical to everything the US was fighting for, and descending into it would show that government of the people was an absurdity. Part of what makes war War is its subordination to politics; it becomes meaningless when the reason behind it ceases to influence it, so they have to be fought with political objectives in mind.

McClellan believed that most southerners were Unionists at heart, and his initial campaign in West Virginia seemed to vindicate his belief that being soft on slavery would have good results; by contrast, Fremont's premature emancipation attempt in Missouri saw the state collapse into a hideous guerrilla quagmire.

In addition to threats to slavery strengthening Southern resistance, it was believed that greater bloodshed would make the Radical Republican faction stronger, and that they would demand harsher terms of reunification. This would in turn inspire greater resistance on the part of the South, leading to even more bloodshed and harsher terms. In this vicious cycle, the only way the war could be won was through despotism and butchery, again wholly antithetical to what Union meant to Americans; McClellan felt he had to balance the pressures of party politics against the true political objective, and that the Abolitionists were as extreme and as much a threat to the Union as the secessionists.

As such, McClellan planned his operations to achieve the policy objectives with minimal cost to the North and minimal existential threat to the South. Time was on the U.S.'s side, in that it could build up a far more powerful army than the South in the forseeable future. This buildup was absolutely necessary, as the pre-war US Army was absolutely incapable of the task of subduing the Southern Confederacy. The army needed to be mustered, gathered, organized, supplied, and trained; from there, it needed to be employed on the best bases and lines of operation to achieve the US's war aims.

When he received command of the army in the Washington defenses, it had just been humiliated at Manassas, and much of its fighting strength would evaporate in the coming weeks as 90 day enlistments expired; by the time any effective army could be organized, it was winter, and winter campaigns were dicey things in the musket era. Not something to take a chance on when representative government itself hung in the balance.

McClellan is usually thought of in the exclusive context of the Eastern Theatre, but as General-in-Chief since November 1861, he had command of Union armies in the West as well, and was responsible for their coordination with the rest of the Union's field forces. It was important for the armies to attack together, to keep the Confederates from shifting forces between threatened regions to deal with threats separately. Much of the delay in Spring 1862 was not because of McClellan's hesitancy, but General Halleck's protests that he wasn't yet ready to move with the rest of the army. The general offensive of Spring 1862 finally got underway in mid February when Halleck let Grant off the leash after hearing a false report that the Confederates were reenforcing the Western theatre.

In early March, Lincoln removed McClellan as General-in-Chief, which was probably a mistake on his part. In many ways, he was a much better General-in-Chief than field commander, and as one of the US's top students of 'big war science,' his hand at the top was critical for keeping the simultaneous offensives on track. Without replacing him, Lincoln shattered the US's unity of command; each army could act as it pleased without direction from a military professional.

This furthermore complicated McClellan's strategy on the Peninsula itself. The Confederates had established a fortified line along the Warwick river on the York-James peninsula; trying to overrun it would likely be bloody, and throw away the US's big advantages. McClellan intended to land further up the Peninsula to turn the Confederates out of position, but independent of his command, the Navy refused to do so unless the guns at Gloucester Point were neutralized, so McClellan conducted siege operations against the Warwick line. He confronted the Confederates with a situation where they could not win. Once McClelllan built up his siege lines and laid his batteries, whatever was on the Warwick line was going to get blasted to Washington's bosom. If they retreated, they would surrender Norfolk and the CSS Virginia to the Union, and would suffer a blow to their morale.

McClellan engaged the Confederates with species of war they could not win, as Joseph E. Johnston put it. McClellan would establish a strong position and use it to take the next one. It's not lightning warfare, but it was getting the job done in the best way possible for the Union's actual war goals.

However, the lack of unity in command derailed the campaign. Their were four separate armies operating in Virginia in 1862: McClellan's, two armies in the Shenandoah Valley, and McDowell's I Corps between the two regions. Lincoln had insisted on a large force to protect Washington, whereas McClellan believed his right flank would be insecure for operations against Richmond without I Corps. Furthermore, it meant he could not command Halleck to take Decatur Alabama, which allowed Albert Sidney Johnson to mass against Grant and almost destroy the main Union army in the West.

When Robert E. Lee took command, the chaos in the Union high command presented him with a great opportunity. Because of Jackson's success in the Valley Campaign, Lincoln believed Washington to be in danger, and under McDowell's protest, ordered I Corps into the Valley. Jackson's army slipped out. The bulk of the Union army was south of the Chickahomminy River, but it's rail supply line began north, and was guarded by Porter's V corps. However, V Corps's flank was not secure without I Corps, so when Lee attacked this isolated corps with most of his army (about equal in numbers with McClellan's whole force, mind; recruiting offices in the north had been closed at the same time the Confederates were instituting a draft), the Army had to abandon it's base of operations on White House and the Richmond York River Railroad and shift to Harrison's Landing on the James.

McClellan intended to renew the offensive against Richmond or Petersburg on the south bank of the James, but he no longer had the confidence of the politicians, and Lincoln was engaged in some wartime party-building. General Pope got the administration's interest by promising to fight a big battle and win a big victory in time for the fall elections, so he received priority support; most of McClellan's army was pulled out from under him, and sent to it's worst defeat of the war at 2nd Bull Run.