Looking specifically at the German invasion of WWII, and the French invasion of the Napoleonic Wars, why did these countries decide to invade the country in the summer? Wouldn't it have made more sense for them to being their invasion at the start of winter, when they were closer to their supply lines, and then by the time they got to the point that the supply line would begin getting a distance away winter would be over, and they'd have 8-9 months to push through Russia, instead of the 4-5 months they gave themselves in both of the presented examples? Or did they have other reasons for invading at that time of year? From a military strategist point, it just doesn't make much logical sense
Looking specifically at the German invasion of WWII, and the French invasion of the Napoleonic Wars, why did these countries decide to invade the country in the summer?
The summer is really the only time of year when heavy equipment can be moved through Russia with any efficiency. In the autumn and spring, there are 1000 kilometers of muddy fields between you and Moscow, in the winter there are 1000 kilometers of snowy fields between you and Moscow. Neither allows for efficient transportation of heavy equipment (the winter technically might but the French and Germans both lacked training, experience and equipment to move large amounts of cargo under Russian winter conditions)
Wouldn't it have made more sense for them to being their invasion at the start of winter, when they were closer to their supply lines, and then by the time they got to the point that the supply line would begin getting a distance away winter would be over, and they'd have 8-9 months to push through Russia, instead of the 4-5 months they gave themselves in both of the presented examples?
If you start an invasion of Russia at the start of winter you're going to see the Russians scratching their heads, pulling back and burning everything in front of you (much like they did in the summer). The difference is, before long you'll be pushing forward with only a few hours of daylight every day, through knee deep snow, in temperatures that will see men dying of hypothermia very fast. Your supply lines will soon get streched anyway because they function very poorly or not at all under those conditions. Come spring, the Russians will simply come back and bury the frozen remnants of an army before it starts to smell.
If you should, by some miracle, manage to winter an army in Russia, you'll be greeted by the lovely Russian spring when every road, field and pasture is transformed into a beautiful muddy swamp if you try to move something heavy through it. In the end, you won't be moving forward without supply lines until summer anyway and whatever land gains you manage before the Russian winter freezes you in place will be insignificant compared to the losses you will suffer to frostbite, hypothermia and starvation.
did they have other reasons for invading at that time of year? From a military strategist point, it just doesn't make much logical sense
As demonstrated above, it is the only time of year and invasion of Russia is feasible.
It actually makes perfect sense. Russia has the rasputitsa season, an extremely muddy time of year. Every spring and fall, roads basically liquifiy into mud, and vehicles, horses, wagons, etc. all get mired. During this time, your advance would be slowed to a crawl, giving the enemy valuable time to bring up reinforcements.
Thus, the only two times to invade are during Summer and Winter. However, in order to avoid the debilitating circumstances of the mud, the entire campaign must be finished before the mud starts. Essentially, it is a choice to try and be in Moscow before winter ends or summer ends. Obviously, most people tend to choose summer.
If you are really interested I recommend this:
Battling the Elements: Weather and Terrain in the Conduct of War
It dedicate chapters exclusively to the invasion of Russia by Hitler, Napoleon and Gustavus Adolphus of Sweden and explains among other things how they were forced to follow similar invasion routes.