It seems USSR had quite an advanced and powerful army prior to WWII and this purge led him to being underprepared for the Nazi invasion/operation barbarossa.
Did he fear opposition in its ranks?
I can’t speak definitively on how advanced and powerful the Red Army was through the 1930s, but maybe someone else can. I address the rest of your question, though.
So, the short answer is, yes, Stalin feared opposition in his ranks. Stalin was a bad guy. I know that as historians, we’re not supposed to make value judgements like that, but every rule has an exception. He was also a short-sighted, paranoid guy who saw enemies everywhere -- including throughout the ranks of the Red Army. The military purge was sparked by the Stalin's perception (and, to a lesser extent, other Soviet leaders) that the Red Army had been deeply infiltrated by foreign agents and that a conspiratorial group was operating within the upper levels of the high command. This wasn't limited to just military officers, though -- hundreds of thousands of people were executed during the Great Terror, and a million or more were run through show trials, convicted, and sent off to labor camps or given some sort of other punitive sentence. The discovery of the ‘military-fascist plot’, which formed a central justification for the military purge, had almost no basis in reality. There was no ‘fifth column’ and no conspiracy in the high command, but the Red Army was purged anyway.(1)
The most common explanation of the military purge continues to emphasize that Stalin wanted to further consolidate his power over the Red Army and that he viewed Marshal Mikhail Tukhachevskii and several other senior officers as potential challengers. Those who support this view have argued that Stalin and the NKVD subsequently, and knowingly, had the group arrested on fabricated charges of participation in a military conspiracy. The officers were put on trial, and all were found guilty of being leading members of a so-called ‘military-fascist plot’ and immediately executed. Stalin then ordered a massive purge of the armed forces to complete this consolidation of power.(2)
Even the head of the Political Administration of the Red Army was in on it: “The evidence of wrecking and espionage [in the Red Army] is not small [...]. We know that Trotskii gave a direct order to his agents from abroad to create a Trotskyist terrorist cell in the Workers’ and Peasants’ Red Army, and Hitler and Trotskii gave an order to organize subversive cells in the Workers’ and Peasants’ Red Army in peace time, which prepared the defeat of the RKKA in the future approaching war.”(3)
Paranoia, plots (for and against), violence, and terror were endemic to the early Soviet system. Lenin himself used repression against perceived enemies of the Bolsheviks as a method of instilling fear and facilitating control, but probably of more relevance to your question: this wasn’t even Stalin’s first purge. Stalin had purged groups before, as he was consolidating power after taking over from Lenin – the (in)famous collectivization and dekulakization campaigns of the early 30s. The Soviet apparatus (on Stalin’s orders) identified and targeted successful peasant-farmers, who were good at growing food, and took their farms and land (turning them into collective farms, where a lot of farmers worked together), starved, and/or killed them. Sometimes all of the above. According to Robert Conquest, if a Soviet bureaucrat showed up to collect the (too high) grain quota, and the peasants did not show signs of hunger (their bodies were not swollen, for example) they were suspected of hiding food.(4) After the brutal collectivization campaign, there was a Soviet famine from 1932-1933, which very well may have been precipitated by the killing, imprisonment, or relocation of an entire class of successful, knowledgeable farmers. In this middle of the famine, the Soviets engaged in a genocide in Ukraine, to punish the Ukrainians for resisting collectivization so strongly.(5)
And don't forget about the Doctor's plot: the anti-Semitic campaign in 1952 and 1953 where a group of predominantly Jewish doctors from Moscow were accused of conspiring to assassinate Soviet leaders. I’ve read that when Stalin was dying in his sauna, the only doctors available for his guards to call were Jewish, and so rather than call a Jewish doctor to treat Stalin, and then potentially face his wrath for attempting to assassinate him, the guards just kind of… let him die. I don’t remember where I read that, though, so take it with a grain of salt.
In short, stuff like this kept happening throughout the reign of Stalin. The purge of the Red Army wasn’t an isolated incident and it contributed greatly to a reduction in the efficacy of the Army, which hindered the Soviets through the initial part of World War II and especially in the wake of the German invasion. There are many facets to that discussion, as well, but I hope this serves to answer your initial question.