Well, between December 1979 to February 1989 (a hair over nine years), the Soviet Union, with the support of a puppet Afghan government, was fighting a large-scale counter-guerrilla war with Afghan mujahideen.

Did this "bankrupt" the Soviet Union? Not really.

The CIA estimated that the Soviet directly spent/lost about 18 billion rubles on the war from 1979 through 1986, of which about 3 billion rubles would have been spent even if they hadn't invaded (i.e. paying soldiers, etc.). And the CIA itself admitted these numbers were probably on the high side. By contrast, the Soviet Union was producing nearly 118 billion rubles worth of consumer goods in 1972.

Of course, the war in Afghanistan created plenty of other economic issues - there were trade sanctions and a brief trade embargo from 1980-1981 by the US against the USSR. It also created growing domestic political and social dissatisfaction with the Soviet regime.

As for the war's overall role in the fall of the USSR, some scholars go so far as the say the war, "was a key factor, though not the only cause, in the breakdown of the Soviet Union." Other writers say the USSR fell apart for other reasons (pressure from the United States and the West, Gorbachev and Yeltsin's liberalizations, a horrifically-inefficient centrally-planned economy, excessive military spending, rising nationalism in the Soviet republics etc. etc.)

I will concur with the earlier answer here. Afghanistan played an indirect role in the fall of the USSR insofar as it and the confrontational Soviet military stance in the Cold War were policies that Gorbachev ultimately sought to change in order to direct resources for domestic reform projects, but it was those domestic reform projects themselves and the forces they unleashed that caused the actual dissolution of the USSR.

An answer I wrote a while back can be found here on Gorbachev's role in the dissolution of the USSR. You also might be interested in the this answer I wrote to a similar question, namely did the SDI program under Reagan lead to Soviet bankruptcy and dissolution (short answer: not really).

Let me pull out a chunk of the latter to contextualize Soviet economic troubles under Gorbachev:

Now, the Soviet economy was facing difficulties by the time that Gorbachev became General Secretary in 1985. The Soviet budget had been running small deficits since the 1970s, defense expenditures made up some 18 percent of national income (although other estimates concluded that something like 20-40 percent of the economy was involved in military production), and investment in consumer goods industries and social programs was lagging.

Khrushchev had considered shifting resources from defense to the consumer sector, but Brezhnev boosted military spending. When Gorbachev came to power, he was willing to cut back on military expenditures in order to free up capital for the civilian economy, although he and those around him would differ on relative priorities and on the need for introducing market mechanisms. However, ultimately, considering the extremely low pay of the largely conscript Soviet military forces, any major savings would have to be made in cutting military industrial production. While all of these debates occurred with SDI on the table, they also occurred with increased Western technological sophistication, stagnating civilian living standards, and a costly and bloody Soviet intervention in Afghanistan.

By 1988, Gorbachev pushed the economic shift forward with a declaration of unilateral defense spending cuts, personnel cuts of half a million, a planned withdrawal of Soviet military forces from Warsaw Pact states, and a “de-ideologization of interstate relations” in a speech to the UN on December 7.

Ironically, it was Gorbachev’s attempt to shift the Soviet economy that led to the increasing fiscal instability of the regime. In order to refocus and modernize industrial production, the Soviet Union needed to import new machine tools from abroad. An increase of importation of machine tools, coupled with a fall in international oil revenues (from 30.9 billion rubles in 1984 to 20.7 billion rubles in 1988) caused a massive increase in the deficit: from some 17-18 billion rubles in 1985 to 48-50 billion rubles in 1986, and rising. This was also coupled by a fall in domestic governmental revenue, as Gorbachev’s anti-alcohol campaign cut sales receipts (a Soviet version of a sales tax) from 103 billion rubles in 1983-1984 to 91.5 billion rubles in 1986. The deficit continued to climb, reaching an estimated 120 billion rubles in 1989 (or 10-12 percent of Soviet GNP). By 1990, no one really knew how large the deficit was in reality, and with increasing political reforms giving greater sovereignty to the Soviet Republics, some three fourths of tax collections were withheld from the center by the Republican governments, leading to an effective bankruptcy of the Soviet government. The Soviet government responded to these deficits by printing money, which in turn caused a sharp rise in inflation, an increased scarcity in goods, and a related decline in living standards. Glastnost (greater media openness) meant that increasingly the government was forced to admit the scale of the economic crisis, and the public was very well aware of the problem.

As economist Marshall Goldman notes:

”Gorbachev’s well-intended but misguided economic strategy was in itself enough to cripple any chance to bring about the economic revitalization he wanted to badly. But the macroeconomic implications of his budget deficit eventually came to have their own impact. Whatever their commitment to socialist economic planning, Soviet officials by 1989 and certainly by 1990 belatedly came to understand that macroeconomics and budget deficits, particularly large ones, do matter. As Gorbachev himself admitted in an October 19, 1990, speech to the Supreme Soviet of the USSR, “We lost control over the financial situation in the country. This was our most serious mistake in the years of perestroika…Achieving a balanced budget today is the number one task and the most important one.” “

In 1990, the Shatalin Plan was proposed, to further cut foreign aid, military expenditures, and the KGB budget in an effort to reign in the deficit and inflation. Other proposed plans called for the cutting of food subsidies. Ultimately, Gorbachev was unable to make a firm decision on any of these proposals, and so the economic situation continued to deteriorate. Confidence in Gorbachev, both from reformers and from conservative hard-liners, fell, until the events of 1991 caused Gorbachev to lose complete control of political events, and ultimately caused the end of the government he stood at the top of.

By the time Gorbachev assumed power in 1985, the USSR was in economic and social stagnation, and was spending ever more of its national income on defense that was not keeping up with technological advances in the West. Nevertheless, the cuts to military spending that did occur were largely unilateral moves on Gorbachev’s part as a key in his plans for economic, political and social restructuring of the USSR. Overall, it was these reforms, their economic mismanagement, Gorbachev’s often hands-off and indecisive leadership style, and increased social, ethnic and political tensions that resulted which caused the fall of the USSR.