Chess Opening Theory/1. e4

1. e4 · King's Pawn Opening.
Undoubtedly, the opening move 1.e4, commonly referred to as the King's Pawn Opening, is widely regarded as the most cunning first move in all levels of chess and was famously favored by the legendary world champion, Bobby Fischer, who hailed it as "best by test." By executing this assertive move, White not only opens up crucial lines for the queen and the king's bishop, but also engages in a strategic battle for dominance over the d5 and f5 squares.
However, it must be noted that claiming control over d5 and f5 is a somewhat ambiguous and abstract notion, since Black has the ability to counteract this by playing 1...e6. In reality, the most vital square that White effectively controls by playing 1.e4 is the e4 square itself. Having a white pawn situated on this square impedes Black's chances of placing their own pawn there, which permits White to develop their g1-knight to f3 without concern of it being forced away by a pawn, resulting in a loss of tempo.
Conversely, Black must be aware that if White advances their e4-pawn to e5, the f6 square will not be a safe haven for Black's g8-knight. The simplest approach for Black to address this dilemma would be to imitate White's move and play 1...e5 as a response.
Generally, openings initiated with 1.e4 are viewed as more aggressive and dynamic in comparison to those with 1.d4, although this is a broad generalization and both players have numerous opportunities to impact the type of position that transpires. It is often contended that 1.e4 positions are easier for novice players to comprehend, with a smaller array of associated variations as opposed to 1.d4.
With a pawn on e4, White's optimal tactic would be to subsequently play d4, resulting in the establishment of a robust and stable "classical" center.
Black's responses.
Games in which Black responds with 1...e5 are called "Open Games", whereas games with Black responding by any other move are known as "Semi-Open Games" (or "Asymmetrical King's Pawn Games"). It's useful to think of Black's responses to 1. e4 as motivated by one of the following counterplans: 
Plan 1.
Plan 1 (pawn on e5, share centre) can be carried out very simply with 1...e5. Black sees what White has and wants the same thing. However, White's argument is that moving first in a symmetrical position is eventually going to favour the player moving first.
Plan 2.
Plan 2 (pawn on d5, share centre) is the motivation behind 1...c6, the Caro-Kann Defence, and 1...e6, the French Defence. If Black tries to put a pawn on d5 immediately, White will capture it, so in order to maintain a pawn on d5 Black needs to be able to "recapture" with a pawn from either c6 or e6.
Plan 3.
Plan 3 (attack White's e pawn) leads to 1...d5, the Scandinavian Defence, and 1...Nf6, the Alekhine Defence.
Plan 4.
Plan 4 (prevent White's e4 &amp; d4) is a pleasant side effect of 1...e5 (mentioned above for Plan 1).
But, with 1...c5, the Sicilian Defence, Black can prevent White's d4 advance and also create an asymmetrical position of attack and counter-attack. The Sicilian is by far the most popular reply to 1. e4 among top players.
Plan 5.
There are multiple ways of carrying out Plan 5 (ignore White's centre, deal with it later). 
In the Pirc Defence, the move 1...d6 prepares the move 2...Nf6. In Alekhine Defence (mentioned above for Plan 3), 1...Nf6 can be met by 2. e5 kicking the knight back. But, in the Pirc, after 1...d6 2. d4 Nf6, the move 3. e5 doesn't work because of 3...dxe5 4. dxe5 Qxd1+ 5. Kxd1 Ng4! forking the pawns on e5 and f2. 
So instead, White's usual move to defend the e-pawn is 3. Nc3. Now, White's d-pawn is vulnerable to the advances 3...c5 or 3...e5 because neither White's e-pawn nor c-pawn can defend it.
Plan 5 also covers a number of fringe options. 
Rare Responses.
Other rare responses include:
Theory table.
1. e4