Who opposed the new Bolshevik regime?

The new Bolshevik rulers of Russia met with fierce opposition from many sides.
Their key military opponents were former czarist officers who numbered in the tens of thousands. Some were monarchists while others supported a democratic republic, but all were united in their readiness to fight the Bolshevik dictatorship, which had forcibly dissolved the democratically elected Constitutional Assembly. Many Cossacks were also deeply conservative and supported resistance to the new Bolshevik regime, which threatened to deprive them of their local autonomy and to seize part of their land. Together, the officers, young student volunteers, and Cossacks formed the White troops led by the czarist generals Kornilov, Kolchak, Denikin, and Wrangel. The White forces received substantial material support from France and England and became the backbone of armed resistance to the Bolsheviks. White armies from the east (Siberia and the Urals), the north (Archangel), and the south (Ukraine, North Caucasus, and the Lower Volga region) attacked the Bolshevik forces, who had to defend the central regions of European Russia.
The Socialist Revolutionaries were the largest political force opposing the Bolsheviks. Theirs was the largest political party in Russia; they had received a majority of votes during the elections to the Constitutional Assembly. The Socialist Revolutionaries were split into the right majority and left minority wings; the left wing of the Socialist Revolutionaries supported the Bolsheviks temporarily and participated in the Bolshevik government. When the Bolsheviks signed the highly unequal Brest-Litovsk Peace Treaty with Germany, however, the Socialist Revolutionaries rebelled against the Bolshevik government. Their rebellion failed.
The largest social force opposing the Bolsheviks was not capitalists, aristocrats, nor the small Russian middle class, but peasants. At the time, peasants made up about 80 percent of the Russian population. Initially, the peasants supported the Bolsheviks, because the Bolsheviks turned land over from the gentry to the peasants, but soon the peasants became angry about Bolshevik anti-market policies and especially about the armed confiscation of grain by Bolshevik troops planning to use it to feed starving city dwellers. In response, many peasants formed their own armed bands organizing hundreds of uprisings in which they attacked Bolshevik forces from the rear.
Another force opposing the Bolsheviks was the highly influential Russian Orthodox Church, which objected to the atheistic policies of the Bolshevik regime that included persecution of church leaders.
Poland, which had recently won independence, also opposed the Bolsheviks and hoped to increase its territory by defeating them. The Polish Army attacked from the west via Belorussia and the western part of Ukraine. Other foreign countries, notably the US, Britain, France, and Japan, sent troops to occupy parts of Russian territory. At the same time, new Menshevik regimes in Georgia and Armenia were also hostile to Bolsheviks.
How did the Bolsheviks survive this formidable, multipronged assault and proceed to defeat and expel their opponents? They had several critical advantages, including a very talented political leadership that could completely rely on a dedicated, well-functioning party machine with hundreds of thousands of loyal members; shorter lines of internal communication; control over most of the Russian military industry; larger artillery; and enthusiastic supporters among workers, soldiers, sailors, and members of ethnic minorities attracted by the prospective of civil equality. While not all of the Bolsheviks' supporters were communists, many were, and they all shared a deep commitment to their cause and were ready to sacrifice their lives for it. In addition, the Bolsheviks managed to gain the trust and collaboration of much of the old Russian officer corps; these officers considered them the only reliable national government.
Bolshevik opponents were deeply divided and failed to offer meaningful reforms to the peasants. After deserting the Bolsheviks in the beginning of the civil war, more and more peasants changed their mind after experiencing the vengeful, rigid, and oppressive rule of the Whites. Hundreds of thousands of peasants who had previously deserted the Red Army rejoined it. By the summer of 1920, the Red Army had about five million soldiers. In other words, it became a mass army that was many times larger than its strongest opponents. In this way, Bolshevik victory became inevitable.