The particle は is pronounced similarly to わ (unlike the rest of the ハ行 kana) when used as a particle; why is this? What historical shifts went on to cause this irregularity?
In Old Japanese (probably before 800 BC), the pronunciation of 「は」 (and indeed the entire ハ行) was PA, but it later changed to FA (more accurately, [ɸa], with a bilabial fricative), and this was the common pronunciation at least up to the 16th century (we know this since early Portuguese transliteration of Japanese words use the letter F where we would use H today).
While the phoneme /p/ became /f/ in isolation or in the beginning of the word, it remained /p/ after a the consonant ん and in between vowels (except in some compounds such as 魔法 まほう). This led to a verb such as 買う (Old Japanese: 買フ kapu) to be read as kau (<- kawu) in Positive form and kawanai (instead of kapanai) in negative form, and also for the particle 「は」 (which regularly appears after a vowel) to be normalized as WA.
Up to the spelling reform of 1946, Historical Kana Usage (歴史的仮名遣 れきしてきかなづかい) was still the standard, so 買う was still written 買フ (Katakana was more commonly used for Okuriganaback then), and 買わない was 買ハナイ. That changed in the reforms, along with many other archaic and confusing spelling, so much that the current Japanese spelling is almost completely phonemic. Only a handful of traditional spelling quirks remained, and using は for the particle that is pronounced WA is one of them.
The sound 「わ」 used to be written は in old kana usage in some cases. Old kana usage was much more irregular than it is nowadays. The old way of writing has kind of stuck in some words.
It's the same thing with the readings for the particles へ and を too - the modern sound 「え」 used to be written as へ in some cases and 「お」 as を. Of course, pronunciation varies and sometimes you can hear a clear difference between お and を for instance, but in all words besides the particles the old pronunciation differences have disappeared as time has passed.