Why is it a struggle to defend the strength of a currency?

[hyom]

using your method, the Asean governments printed money to defend against their currency's strength. what would happen? There would be a flush of Thai Baht circulating in Thailand only, which could easily cause asset bubble and high inflation.

Having a strong currency, it would create an easy environment of cheaper foreign borrowing, which could encourage local firms to borrow more from international markets in USD(which happened in 1997 in Indonesia at least).

That is a good answer. Let me rephrase in my own words. Although central banks can easily defend against a currency from strengthening by printing money, they have to worry about unintended consequences like creating asset bubbles and inflation. The bust after the boom created by loose monetary policies will be much more devastating if too much money is printed.

the lead to Asian Financial Crisis is failure to defend the strength of Thai Baht/Indonesian Rupiah, etc.

The failure was caused by the fixed currency peg to the US dollar. The Asian currency was artificially strong and was not allowed to fluctuate normally due to the peg. Meanwhile, interest rates in the local currency were high because they reflected the true risk. Businessmen borrowed money in foreign currencies at low interest rates thinking that their central banks will have the means to maintain the currency peg. Unfortunately, a fixed currency peg is hard to maintain if the local currency is over-valued. Although a central bank can prevent its local currency from going up by printing money, it cannot prevent its currency from going down without sufficient foreign reserves. No central bank, not even the Fed, can fight the forex market because it is the most liquid market among all asset classes.