I have usually found the Marshall Attack in the Ruy Lopez to be very potent and very hard to counter.
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1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Be7 6. Re1 b5 7. Bb3 O-O 8. c3 d5
Are there any proven defensive lines or variations to use against this? Just point me in the right direction, please.
A huge amount of theory has just been discovered in the Marshall Attack (I'm assuming you mean the line in the Ruy Lopez). However, this variation has recently fallen out of practice among top level GMs because white has been hard-pressed to find decent replies to the most critical lines. Basically, I would recommend playing one of the anti-Marshall games (8. h3 has been popular) unless you want to spend days studying opening theory to reach a suspect position with white.
Wikipedia has an article on the Marshall attack which is worth reading.
The "classic" defense against the Marshall Attack was waged against Marshall himself by Jose Raul Capablanca, the future world champion, in 1918. See this article about the game.
Since then, lines have evolved on both sides, but the underlying principles remain the same. Master the one game, and you'll go a long way to mastering the defense as a whole.
I'm posting this because this year some very interesting games have been played showing new ideas just discovered for White which can give an idea of how difficult it is to prepare especially at the very top level.
The first game that I want to mention is the game between Ivanchuk and Wesley So played this year in Wijk aan Zee. So played a new line (well known to the top players thanks to an article published by Anish Giri in New in Chess). Ivanchuk didn't know the line and got crushed. You can see an analysis of the game by the GM Jan Gustafsson here.
The second one is one played in the final of the World Cup in Baku between Peter Svidler and Sergey Karjakin which also displayed a very interesting line. It was a blitz game where Svidler got a huge advantage thanks to his better preparation but finally lost due to blundering a rook.
To sum up, if you're interested I recommend you the series of videos made by Jan Gustafsson for Chess 24 called Latest Trends on the Marshall where you can find some very interesting ideas for both Black and White like the two mentioned above.
